Stable radicals during photodecarbonylations of trityl-alkyl ketones enable solid state reactions through primary and secondary radical centers.

The solid state photoexcitation of several triphenylmethyl-alkyl ketones resulted in the loss of CO and the exclusive formation of radical-radical combination products. Differences in reactivity suggest a stepwise mechanism with the unprecedented formation of primary and secondary radicals in some of the radical pair intermediates in the solid state.

Monitoring structural transformations in crystals. 11. Yang photocyclizations--one type of reaction, but diversity of structural changes.

Structural changes proceeding in a crystal during the Yang photocyclization of the salt 6,6-diethyl-5-oxo-5,6,7,8-tetrahydronaphthalene-2-carboxylate with (1S)-1-(4-methylphenyl)ethylamine were monitored by means of X-ray structure analysis. The course of the photoreaction was evaluated on the basis of the geometrical parameters for the pure reactant crystal. Variations in the cell constants, the product content, the geometry of the reaction centre, the orientation of molecular fragments and the geometry of hydrogen bonds were described and analyzed. It was found that the cell volume increased until 56% product content and decreased thereafter. The distance between the directly reacting C atoms was constant, approximately 3.0 A, until approximately 75% reaction progress. Analysis of the distance between atoms that would participate in the formation of the second (unobserved) enantiomorph excluded the formation of such an isomer. Molecular fragments varied their orientation during the photoreaction, and the largest change was observed for the carboxylate group despite its participation in strong hydrogen bonds. The geometry of the hydrogen bonds changed during the photoreaction. The largest change was 0.17 A for the D...A distance and 13 degrees for the D-H...A angle. A comparison of the intra- and intermolecular parameters for the studied salt with data for other compounds undergoing the Yang photocyclization in crystals revealed a diversity of structural changes brought about by this type of photochemical reaction.

Genetic mapping of sulfur assimilation genes reveals a QTL for onion bulb pungency.

Onion exhibits wide genetic and environmental variation in bioactive organosulfur compounds that impart pungency and health benefits. A PCR-based molecular marker map that included candidate genes for sulfur assimilation was used to identify genomic regions affecting pungency in the cross 'W202A' x 'Texas Grano 438'. Linkage mapping revealed that genes encoding plastidic ferredoxin-sulfite reductase (SiR) and plastidic ATP sulfurylase (ATPS) are closely linked (1-2 cM) on chromosome 3. Inbred F(3) families derived from the F(2 )population used to construct the genetic map were grown in replicated trials in two environments and bulb pungency was evaluated as pyruvic acid or lachrymatory factor. Broad-sense heritability of pungency was estimated to be 0.78-0.80. QTL analysis revealed significant associations of both pungency and bulb soluble solids content with marker intervals on chromosomes 3 and 5, which have previously been reported to condition pleiotropic effects on bulb carbohydrate composition. Highly significant associations (LOD 3.7-8.7) were observed between ATPS and SiR Loci and bulb pungency but not with bulb solids content. This association was confirmed in two larger, independently derived F(2) families from the same cross. Single-locus models suggested that the partially dominant locus associated with these candidate genes controls 30-50% of genetic variation in pungency in these pedigrees. These markers may provide a practical means to select for lower pungency without correlated selection for lowered solids.

Structure-reactivity correlations for solid-state enantioselective photochemical reactions established directly from powder X-ray diffraction.

A prerequisite for the development of structure-reactivity correlations for photoreactive crystalline materials is to have detailed knowledge of the structural properties of the reactant crystalline phase. In some cases, however, the materials of interest can be prepared only as microcrystalline powders and are not amenable to structural characterization by single-crystal X-ray diffraction. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, leading to the development of structure-reactivity correlations. In particular, a series of three photoreactive organic salts are considered, which undergo the same photochemical asymmetric reaction but with high enantiomeric excess in two cases and low enantiomeric excess in the other case. The structural properties of the three salts determined from powder X-ray diffraction data are shown to provide a direct rationalization of these observations.

Monitoring structural transformations in crystals. 8. Monitoring molecules and a reaction center during a solid-state Yang photocyclization.

Structural changes taking place in a crystal during an intramolecular photochemical reaction [the Yang photocyclization of the alpha-methylbenzylamine salt with 1-(4-carboxybenzoyl)-1-methyladamantane] were monitored step-by-step using X-ray structure analysis. This is the first example of such a study carried out for an intramolecular photochemical reaction. During the photoreaction, both the reactant and product molecules change their orientation, but the reactant changes more rapidly after the reaction is about 80% complete. The distance between directly reacting atoms in the reactant molecule is almost constant until about 80% reaction progress and afterwards decreases. The torsion angle defined by the reactant atoms that form the cyclobutane ring also changes in the final stages of the photoreaction. These phenomena are explained in terms of the influence of many product molecules upon a small number of reacting molecules. The adamantane portion shifts more than the remaining part of the anionic reactant species during the reaction, which is explained in terms of hydrogen bonding. The structural changes are accompanied by changes in the cell constants. The results obtained in the present study are compared with analogous results published for intermolecular reactions.

Genetic mapping of a major gene affecting onion bulb fructan content.

The non-structural dry matter content of onion bulbs consists principally of fructose, glucose, sucrose and fructans. The objective of this study was to understand the genetic basis for the wide variation observed in the relative amounts of these carbohydrates. Bulb carbohydrate composition was evaluated in progeny from crosses between high dry matter storage onion varieties and sweet, low dry matter varieties. When samples were analysed on a dry weight basis, reducing sugar and fructan content exhibited high negative correlations and bimodal segregation suggestive of the action of a major gene. A polymorphic SSR marker, ACM235, was identified which exhibited strong disequilibrium with bulb fructan content in F(2:3) families from the 'W202A' x 'Texas Grano 438' mapping population evaluated in two environments. This marker was mapped to chromosome 8 in the interspecific population 'Allium cepa x A. roylei'. Mapping in the 'Colossal Grano PVP' x 'Early Longkeeper P12' F2 population showed that a dominant major gene conditioning high-fructan content lay in the same genomic region. QTL analysis of total bulb fructan content in the intraspecific mapping population 'BYG15-23' x 'AC43' using a complete molecular marker map revealed only one significant QTL in the same chromosomal region. This locus, provisionally named Frc, may account for the major phenotypic differences in bulb carbohydrate content between storage and sweet onion varieties.

Photochemistry of 1-isopropylcycloalkyl aryl ketones: ring size effects, medium effects, and asymmetric induction.

[reaction: see text] The n = 0, 1, and 2 ketones shown above undergo Yang photocyclization in solution, but only the n = 1 analogues react this way in the solid state. Based on X-ray crystallography, these differences in reactivity are attributed to an unusually large distance for 1,4-hydroxybiradical cyclization in the solid state for the n = 0 and 2 ring systems, which leads to predominant reverse hydrogen transfer (rht). Enantiomeric excesses of up to 99% can be achieved in the case of the n = 1 system through the use of the solid-state ionic chiral auxiliary method of asymmetric synthesis.

Asymmetric synthesis of dihydrofurans via a formal retro-Claisen photorearrangement.

Solution-phase irradiation of a series of syn-7-benzoylnorbornene derivatives is shown to lead to cis-fused dihydrofuran derivatives in low quantum but excellent chemical yields in what is formally a retro-Claisen rearrangement. In analogy to the well-known Paterno-Buchi reaction, the first step of the rearrangement is suggested to involve (n,pi)(3)-mediated addition of the carbonyl oxygen to the norbornene double bond, producing a triplet 1,4-biradical. This intermediate, rather than closing to the oxetane, undergoes cleavage accompanied by intersystem crossing to form the dihydrofuran. To determine whether the retro-Claisen photorearrangement could be carried out enantioselectively, the 7-benzoylnorbornene reactant was equipped with a para-carboxylic acid substituent to which a series of optically pure amines was attached ionically via salt bridges. Irradiation of these salts in the crystalline state followed by diazomethane workup (the solid-state ionic chiral auxiliary method) was shown to afford the corresponding dihydrofuran in optical yields as high as 93% at 95% conversion. X-ray crystallography revealed that the enantioselectivity arises from crystallization of the reactant in a conformation in which the carbonyl oxygen is more favorably oriented for bond formation to one end of the norbornene double bond than the other, thus leading to a predominance of a single enantiomer.

Enantioselective photochemical synthesis of a simple alkene via the solid state ionic chiral auxiliary approach.

Irradiation of cis-bicyclo[4.3.0]non-8-ylacetophenone derivatives (1) in solution and the solid state yields cis-3a,4,5,6,7,7a-hexahydro-1H-indene (2) via a Norrish type II cleavage process. Asymmetric induction studies were conducted by providing the reactants with carboxylic acid substituents to which ionic chiral auxiliaries were attached through salt formation with optically pure amines. Irradiation of the salts (5 in total) in the crystalline state gave enantiomeric excesses of up to 44%. Single-crystal X-ray diffraction studies were performed on ketone 1a as well as salts 1d and 1g, and on this basis, the structure-reactivity relationships involved are discussed.

1,4-hydroxybiradical behavior revealed through crystal structure-solid-state reactivity correlations.

Structure-reactivity correlations for triplet 1,4-hydroxybiradicals in solution are made difficult by the presence of multiple reactive conformers and the possibility of conformation-dependent intersystem crossing. These problems can be overcome by working in the crystalline state, where the conformations of the 1,4-hydroxybiradicals are fixed and determinable by X-ray crystallography of the parent ketones, assuming that hydrogen atom abstraction occurs with little or no change in conformation. This approach is applied to 15 bi- and tricyclic ketones designed to have slightly different biradical conformations, so that the effect of small and incremental changes in geometry on biradical behavior can be tested. The results indicate that, while geometry does have a strong influence on 1,4-hydroxybiradical partitioning between cyclization, cleavage, and reverse hydrogen transfer, a full understanding of the results requires that the strain involved in forming the cyclization products be taken into account.

Asymmetric synthesis in a Norrish type II cleavage reaction induced by crystal chirality.

Moderate to high enantiomeric excesses for the cis and trans olefinic products of a Norrish type II cleavage reaction have been obtained for the first time through the use of the solid-state ionic chiral auxiliary approach.

Thermal and photochemical transformation of conformational chirality into configurational chirality in the crystalline state.

Because they crystallize in chiral conformations in which abstraction of only one of two diastereotopic gamma-hydrogen atoms is possible, salts formed between achiral keto-acids possessing the tricyclo[4.4.1.0]undecane ring system and optically pure amines undergo Norrish type II cleavage in the solid state in enantiomeric excesses as high as 95% at 98% conversion, following removal of the ionic chiral auxiliaries. Thermal enolene rearrangement of the same salts results in optical yields approximately half those observed for the photochemical reaction.

Enhanced enantio- and diastereoselectivity via confinement and cation binding: yang photocyclization of 2-benzoyladamantane derivatives within zeolites.

Irradiation of 2-benzoyladamantane derivatives in zeolites yields the endo-cyclobutanols as the only photoproduct via a gamma-hydrogen abstraction process. The cyclobutanols readily undergo retro-aldol reaction to give delta-ketoesters. The enantiomeric excess (ee) in the endo-cyclobutanols is measured by monitoring the ee in the ketoesters. Whereas in solution the ee in the product ketoester is zero, within achiral NaY zeolite, in the presence of a chiral inductor such as pseudoephedrine, ee's up to 28% have been obtained. The influence of zeolite on several chiral esters of 2-benzoyladamantane-2-carboxylic acids has also been examined. Whereas in solution the diastereomeric excess is <15%, in zeolite the delta-ketoesters are obtained in 79% de (best examples). Ab initio computations suggest that enhancement of chiral induction within zeolites is likely to be due to cation complexation with the reactant ketone. Alkali ion-organic interaction, a powerful tool, is waiting to be fully exploited in photochemical and thermal reactions. In this context zeolites could be a useful medium as one could view them as a reservoir of "naked" alkali ions that are only partially coordinated to the zeolite walls.

Confined space and cations enhance the power of a chiral auxiliary: photochemistry of 1,2-diphenylcyclopropane derivatives.

Alkali ion-exchanged Y-zeolites significantly enhance asymmetric induction in the photoisomerization of a number of cis-1,2-diphenylcyclopropane derivatives containing a distant chiral auxiliary.

Enhanced regioselectivity of Yang photocyclization in the crystalline state.

[reaction: see text] In contrast to the solution state, where Yang photocyclization of ketones of general structure 1 leads to equal amounts of cyclobutanols 2 and 3 (by abstraction of HA and HB, respectively), irradiation in the crystalline state is much more regioselective, favoring either 2 or 3 depending on the nature of the substituent X. X-ray crystallography and molecular mechanics calculations reveal the source of this remarkable solid-state regioselectivity.

1-Methyl-5,6-diphenylpyrazine-2(1H)-thione.

The title compound, C(17)H(14)N(2)S, crystallizes in a triclinic unit cell, with two crystallographically independent molecules in the asymmetric unit. The two independent molecules pack in the same sense and form segregated layers along the c axis. The crystal is light-stable and no dimers are formed under irradiation. The intermolecular distances between the potential reactive centers (the C-3 and C-5 ring positions) are 4.093 (4) and 5.643 (4) A for molecule A, and 4.081 (4) and 5.614 (4) A for molecule B.