Access to oxetane-containing psico-nucleosides from 2-methyleneoxetanes: a role for neighboring group participation?

The first psico-oxetanocin analogue of the powerful antiviral natural product, oxetanocin A, has been readily synthesized from cis-2-butene-1,4-diol. Key 2-methyleneoxetane precursors were derived from ß-lactones prepared by the carbonylation of epoxides. F(+)-mediated nucleobase incorporation provided the corresponding nucleosides in good yield but with low diastereoselectivity. Surprisingly, attempted exploitation of anchimeric assistance to increase the selectivity was not fruitful. A range of 2-methyleneoxetane and related 2-methylenetetrahydrofuran substrates was prepared to explore the basis for this. With one exception, these substrates also showed little stereoselectivity in nucleobase incorporation. Computational studies were undertaken to examine if neighboring group participation involving fused [4.2.0] or [4.3.0] intermediates is favorable.

Catalytic double carbonylation of epoxides to succinic anhydrides: catalyst discovery, reaction scope, and mechanism.

The first catalytic method for the efficient conversion of epoxides to succinic anhydrides via one-pot double carbonylation is reported. This reaction occurs in two stages: first, the epoxide is carbonylated to a beta-lactone, and then the beta-lactone is subsequently carbonylated to a succinic anhydride. This reaction is made possible by the bimetallic catalyst [(ClTPP)Al(THF)2]+[Co(CO)4]- (1; ClTPP = meso-tetra(4-chlorophenyl)porphyrinato; THF = tetrahydrofuran), which is highly active and selective for both epoxide and lactone carbonylation, and by the identification of a solvent that facilitates both stages. The catalysis is compatible with substituted epoxides having aliphatic, aromatic, alkene, ether, ester, alcohol, nitrile, and amide functional groups. Disubstituted and enantiomerically pure anhydrides are synthesized from epoxides with excellent retention of stereochemical purity. The mechanism of epoxide double carbonylation with 1 was investigated by in situ IR spectroscopy, which reveals that the two carbonylation stages are sequential and non-overlapping, such that epoxide carbonylation goes to completion before any of the intermediate beta-lactone is consumed. The rates of both epoxide and lactone carbonylation are independent of carbon monoxide pressure and are first-order in the concentration of 1. The stages differ in that the rate of epoxide carbonylation is independent of substrate concentration and first-order in donor solvent, whereas the rate of lactone carbonylation is first-order in lactone and inversely dependent on the concentration of donor solvent. The opposite solvent effects and substrate order for these two stages are rationalized in terms of different resting states and rate-determining steps for each carbonylation reaction.

Ligand-dependent catalytic cycle and role of styrene in nickel-catalyzed anhydride cross-coupling: evidence for turnover-limiting reductive elimination.

Results from a mechanistic study on the Ni(COD)2-bipy-catalyzed alkylation of anhydrides are consistent with turnover-limiting reductive elimination at high Et2Zn concentrations. While the presence of styrene does not affect the initial rate of alkylation, it appears to inhibit catalyst decomposition and provides higher product yield at long reaction times. In contrast, Ni(COD)2-iPrPHOX-catalyzed anhydride alkylation proceeds through two competing catalytic cycles differentiated by the presence of styrene. The presence of styrene in this system appears to accelerate rate-limiting oxidative addition and promotes the cycle which proceeds 4 times more rapidly and with much higher enantioselectivity than its styrene-lacking counterpart.