A pyranose ring contraction
of ethyl 1-thio-β-d-galactopyranosides has been discovered
that proceeds with retention of aglycon under mildly acidic conditions
(aq TFA in CH2Cl2). Key factors for success
of this rearrangement are the presence of bulky silyl (TIPS or TBDPS)
substituents at both O-2 and O-3 and a free hydroxy
group at C-4 (derivatives with acid-labile protective groups at O-4
will also engage in this reaction). The rearrangement cleanly proceeds
for 2,3-di-O-TIPS derivatives with two hydroxy groups
at C-4 and C-6, acid-labile TES groups at O-4 and O-6, or one acyl
substituent (Bz, ClAc) at O-6. A possibility to switch the direction
of the debenzylidenation reaction in 4,6-O-benzylidene-2,3-di-O-TIPS/TBDPS derivatives by the choice of an acid (TFA,
which cleanly gives furanose, versus AcOH, which cleaves benzylidene
acetal only) may present an advantage in the divergent synthesis of
selectively protected glycosyl donors (either in furanose or pyranose
form) useful for the synthesis of biologically important oligosaccharides.
An approach to the synthesis of seven-membered systems via the chain elongation of nucleophilic propenes and subsequent 8π-electrocyclization is proposed. The cascade reaction yields either cycloheptadienes or bicycloheptenes, and the latter are formed via a 6π-electrocyclization of intermediate cycloheptadienyl anion which was proved to be reversible in a basic medium. The electrocyclic nature of the ring-closing reactions was supported by density functional theory and DLPNO/CCSD(T) calculations. Highly electron-deficient cycloheptatrienes can be obtained from cycloheptadienes or bicycloheptenes via oxidation either introduced into the cascade reaction or performed as a separate reaction, with the overall yield of up to 81%. The oxidation step was performed by means of a rarely encountered Cu(II)-catalyzed dehydrogenation of cycloheptadienes or bicycloheptenes, and so the reaction mechanism was proposed. Stable formally 8π-antiaromatic cycloheptatrienyl-anion containing compounds were obtained, and some correlations between their UV−vis spectra and the structure of the distorted cycloheptatrienyl-anion moiety were clarified. Additionally, a base-induced retro-[2 + 2]-cycloaddition in a bicycloheptene derivative gave cyanotetra-(methoxycarbonyl)cyclopentadienyl cesium.
We report the first total synthesis of racemic Odontosyllis
undecimdonta luciferin, a thieno[3,2-f]thiochromene
tricarboxylate comprising a 6-6-5-fused tricyclic skeleton with three
sulfur atoms in different electronic states. The key transformation
is based on tandem condensation of bifunctional thiol-phosphonate,
obtained from dimethyl acetylene dicarboxylate, with benzothiophene-6,7-quinone.
The presented convergent approach provides the synthesis of the target
compound with a previously unreported fused heterocyclic core in 11
steps, thus allowing for unambiguous confirmation of the chemical
structure of Odontosyllis luciferin by 2D-NMR spectroscopy.
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