Ring-Closing Metathesis in Carbohydrate Annulation.

Even eight-membered rings (such as in 2) can be formed by ring-closing metathesis of glucose derivatives such as 1. Enantiomerically pure tricyclic spiro compounds can also be prepared.

Toward a general strategy for the synthesis of 3,4-dihydroxyprolines from pentose sugars.

A general strategy is proposed, wherein a pentose sugar gamma-lactone can be converted, via a series of nine reactions, to a 3,4-dihydroxyproline, suitably protected for use in peptide synthesis. Thus, D-ribonolactone (6) has been converted to N-fluorenylmethoxycarbonyl-3,4-di-O-tert-butyldimethylsilyloxy-D-2,3-cis-3,4-cis-proline (7) in 18.9% overall yield. Likewise, L-arabinonolactone (11) has been converted to N-fluorenylmethoxycarbonyl-3,4-di-O-tert-butyldimethylsilyloxy-L-2,3-cis-3,4-trans-proline (36) in 13.7% overall yield and L-lyxonolactone (12) to N-fluorenylmethoxycarbonyl-3,4-di-O-tert-butyldimethylsilyloxy-L-2,3-trans-3,4-cis-proline (37) in 11.2% overall yield. These building blocks have also been fully deprotected to give the free amino acids. We believe that this series of reactions ought to be applicable to the synthesis of any of the eight stereoisomers of 3,4-dihydroxyproline, by judicious selection of the pentose starting material.

The synthesis of fused and spiro annulated carbohydrate structures using copper(I) catalysed intramolecular photoannulation of glucose derivatives.

Intramolecular [2 + 2] photoannulation catalysed by copper(i)triflate has been applied to a series of carbohydrate derivatives obtained from glucose. Dienes 1a and 1b lead to cyclobutanes 3a and 3b whereas the diastereoisomeric dienes 5a and 5b gave diastereoisomeric products 7a and 7b. These results demonstrate that the reaction is stereospecific. Products 3a and 7a were converted into bromoesters 4 and 9 respectively. The Vasella elimination of 8 lead to the expected bicyclic aldehyde 10 and the ring expanded hydroxy ketone 12. The stereospecific formation of enantiomerically pure spiro annulated carbohydrates 18a and 18b was demonstrated whereas in example 19 no selectivity in the formation of 20 and 21 was observed.