Synthesis and reactivity of nonstabilized diazo sugars

Synthesis of All Configurational Isomers of 1,6-Anhydro-2,3,4-trideoxy-2,3-epimino-4-fluoro-β-d-hexopyranoses

“…The structures of products 27 and 28 were determined by X-ray analysis. A similar rearrangement also took place on methanolysis of diazo compound 29 and yielded methyl pyranoside 30 . The formation of these rearrangement products is consistent with initial formation of C-4 carbocationic species followed by 1,2-alkyl shift, as depicted in Scheme .…”

“…The stereochemical outcome of this reaction was attributed to a concerted process with synchronous formation of C-4 carbocation, a shift from C-6 to C-4, and an equatorial attack of the nucleophile at C-5. 16 The 1,2-alkyl shift involved in formation of products 27 and 28 seems to indicate that with some substrates the fluorination with DAST should follow a S N 1 cationic mechanism rather than the otherwise more usual S N 2 mechanism. 17 The S N 2-type displacement reactions at C-4 of mannose derivative 26 are probably suppressed because of the steric hindrance exerted by the substituents at the β-face of the substrate.…”

“…It is noteworthy that the major product 27 of reaction with DAST has the same configuration at C-9 (C-5 in numbering of 26) as the product of methanolysis 30. The stereochemical outcome of this reaction was attributed to a concerted process with synchronous formation of C-4 carbocation, a shift from C-6 to C-4, and an equatorial attack of the nucleophile at C-5 16. The 1,2-alkyl shift involved in formation of products 27 and 28 seems to indicate that with some substrates the fluorination with DAST should follow a S N 1 cationic mechanism rather than the otherwise more usual S N 2 mechanism 17.…”

Fluorine effect in nucleophilic fluorination at C4 of 1,6-anhydro-2,3-dideoxy-2,3-difluoro-β-D-hexopyranose

“…Also, we did not observe any 1,2-alkyl shift of C6 from C5 to C4 with enlargement of the dioxolane ring followed by fluorine introduction at C5 (compound 20, step e) [33]. This known alkyl shift [29,39] is usually displayed by more carbocationic character of the intermediate species [40]. In our case, the polyfluoroalkyl group could destabilize the adjacent carbocation center [41][42][43], avoiding 1,2-alkyl shift, and thus formation of byproduct 20.…”