Concise total asymmetric syntheses of (−)-fagomine, two of its epimers, and two seven-membered ring congeners

Glycal mediated synthesis of piperidine alkaloids: fagomine, 4-epi-fagomine, 2-deoxynojirimycin, and an advanced intermediate, iminoglycal

Glucal and galactal are transformed into 2-deoxyglycolactams, which are important building blocks in the synthesis of biologically active piperidine alkaloids, fagomine and 4-epi-fagomine. In one of the strategies, reduction of 2-deoxyglycolactam-N-Boc carbonyl by lithium triethylborohydride (Super-Hydride®) has been exploited to generate lactamol whereas reduction followed by dehydration was utilized as the other strategy to functionalize the C1-C2 bond in the iminosugar substrate. The strategies provide the formal synthesis of 2-deoxynojirimycin, nojirimycin and nojirimycin B. DFT studies were carried out to determine the reason for the failure of the formation of the 2-deoxygalactonojirimycin derivative. Further, DFT studies suggest that phenyl moieties of protecting groups and lone pairs of oxygen in carbamate group plays a vital role in deciphering the conformational space of the reaction intermediates and transition-state structures through cation-π or cation-lone pair interactions. The influence of these interactions is more pronounced at low temperature when the entropy factor is small.

Homoallylic amines as efficient chiral inducing frameworks in the conjugate addition of amides to α,β-unsaturated esters. An entry to enantio-enriched diversely substituted amines

The diastereoselective conjugate addition of secondary homoallylamines, obtained in the enantioenriched form via allylmetallation of imines, to α,β-unsaturated esters is reported. This method allows access to valuable building blocks as well as heterocyclic skeletons, providing tertiary amines bearing two chains integrating a stereogenic center adjacent to the nitrogen atom.

Ring closing metathesis (RCM) approach to the synthesis of conduramine B-2, ent-conduramine F-2, aminocyclopentitol and trihydroxyazepane

The syntheses of conduramine B-2, ent-conduramine F-2, aminocyclopentitol and trihydroxyazepane were accomplished from a common precursor, through a divergent approach using ring closing metathesis (RCM) as the key step. Tri-O-benzyl-d-glucal was converted to 3,4,6-tri-O-benzyl-1,2-dideoxy-2-iodo-1-p-toluenesulfonamido-α-d-mannose. Exposure to NaBH4 in MeOH resulted in a facile 1,2-transposition of the -NHTs group with concomitant glycosylation to give methyl 3,4,6-tri-O-benzyl-2-deoxy-2-p-toluenesulfonamido-β-d-glucoside, which was converted into methyl 6-deoxy-6-iodo-glucoside in three steps. Zinc-mediated Vasella's rearrangement proceeded smoothly to give the pluripotent formyl-olefin, possessing both electrophilic and nucleophilic sites, which was used as a common precursor in our diversity-oriented approach. Vinylation of the carbonyl group followed by RCM and subsequent deprotection resulted in the successful synthesis of conduramine B-2 and ent-conduramine F-2 for the first time. On the other hand, the Wittig reaction of the formyl-olefin affords the diene that undergoes Grubbs' I catalyzed RCM and deprotection/reduction to provide 3-amino-cyclopentan-1,2-diol. Utilizing the nucleophilic site at the nitrogen of the common precursor, base mediated N-allylation was carried out to obtain the corresponding diene that underwent a smooth RCM to afford trihydroxyazepane.

Dimethylamine adducts of allylic triorganoboranes as effective reagents for Petasis-type homoallylation of primary amines with formaldehyde

Dimethylamine adducts of triallyl-, triprenyl- and trans-cinnamyl(dipropyl)borane are effective reagents for mild homoallylation of primary amines with aqueous formaldehyde in MeOH without an inert atmosphere. A new concept is proposed for the explanation of the high stability of allylborane-amine adducts in aqueous MeOH.