Anomalous nuclear Overhauser effects in carbon-substituted aziridines: scalar cross-relaxation of the first kind.

Anomalous NOESY cross-peaks that cannot be explained by dipolar cross-relaxation or chemical exchange are described for carbon-substituted aziridines. The origin of these is identified as scalar cross-relaxation of the first kind, as demonstrated by a complete theoretical description of this relaxation process and by computational simulation of the NOESY spectra. It is shown that this process relies on the stochastic modulation of J-coupling by conformational transitions, which in the case of aziridines arise from inversion at the nitrogen center. The observation of scalar cross-relaxation between protons does not appear to have been previously reported for NOESY spectra. Conventional analysis would have assigned the cross-peaks as being indicative of a chemical exchange process occurring between correlated spins, were it not for the fact that the pairs of nuclei displaying them cannot undergo such exchange.

Lithiation-electrophilic substitution of N-thiopivaloylazetidine.

The fourth protocol: the rarely studied N-thiopivaloyl group plays a crucial role in mediating efficient α lithiation and incorporation of diverse electrophiles onto an azetidine ring; in the presence of chiral ligands, this chemistry also provides the first example of an enantioselective electrophilic substitution on a four-membered ring.

Asymmetric synthesis of terminal N-tert-butylsulfinyl aziridines from organoceriums and an alpha-chloroimine.

Addition of N-(2-chloroethylidene)- tert-butylsulfinamide to organocerium reagents in DMPU/THF (1:10) at -78 degrees C followed by warming to 25 degrees C provides terminal N-tert-butylsulfinyl aziridines in good yields (63-92%, nine examples) and diastereomeric ratios (85:15- >99:1).

Tethered aminohydroxylation: dramatic improvements to the process.

[reaction: see text] Changing the identity of the N leaving group on a hydroxylamine-based reoxidant gives a dramatic improvement to the tethered aminohydroxylation reaction. Using OCOC6F5 as a leaving group means that only 1 mol % of osmium is required and yields as high as 98% can be obtained. Acyclic homoallylic alcohols were substrates considered too unreactive for effective use in the tethered aminohydroxylation reaction; improved reaction conditions mean that they have now become viable substrates for oxidation.

Consecutive alkene cross-metathesis/oxonium ylide formation-rearrangement: synthesis of the anti-HIV agent hyperolactone C.

Alpha-diazo-beta-ketoesters bearing allylic ether functionality undergo highly stereoselective Ru-carbene-catalyzed alkene cross-metathesis followed by Rh(2)(OAc)(4)-catalyzed oxonium ylide formation/[2,3] sigmatropic rearrangement in a one-flask operation and in a highly diastereoselective manner. The methodology has been demonstrated in a concise synthesis of the anti-HIV agent hyperolactone C.