Exploitation of sugar ring flipping for a hinge-type tether assisting a [2 + 2] cycloaddition

Abstract

Methyl 3-O-p-methoxybenzyl-β-D-xylopyranoside (2) was exploited as a novel hinge-type tether for the [2 + 2] cycloaddition of cinnamate. The major ring conformation occupied by the 2,4-dicinnamate derivative of 2 was ⁴C₁, which extends two cinnamates along a diequatorial orientation. However, 3-O-deprotected dicinnamate 5, when in a non-polar solvent, favours the ¹C₄ conformation, which assists the approach of two cinnamates with the 1,3-diaxial scaffold. Photoirradiation of compound 5 at 313 nm in CHCl₃ afforded the intramolecular cycloaddition of cinnamates to give methyl β-, δ-, and ξ-truxinates in a 86 : 8 : 6 ratio after transesterification with methanol. The regio- and stereoselectivities are comparable to those reported by others for tethered cinnamates. The per-deuterated dicinnamate derivative of 5 facilitated the conformation analyses of the pyranoside rings by ¹H NMR, indicating that all the products of photoirradiation had ¹C₄-fixed pyranosides. Excellent β-selectivity was achieved when m-bromocinnamate was subjected to hinge-tethered [2 + 2] cycloaddition.