Reductive opening of α-methylspiroketals

Abstract

Syntheses of 5-methyl-1,7-dioxaspiro[5.5]undecane 4 and its three congeners 5, 6, and 7 (α-methylspiroketals), and their reductive ring opening using aluminum hydride or silane - Lewis acid system have been investigated. Each spiroketal was synthesized through stereocontrolled acetalization with 42 – 96% diastereomeric excess (de). The diisobutylaluminum hydride reduction of α-methylspiroketals proceeded via the tight oxocarbenium ion pair complex wherein the CO bond which located at the opposite site against the C(α) - methyl bond was cleaved, affording a configuration-retentive product with 50 – 100% de. The regioselectivity in the silane - Lewis acid reduction of the simplest monomethylspiroketal 4 was controlled by the equilibration of the two possible oxocarbenium inonic intermediates. On the other hand, dimethylspiroketal 5 and siloxyspiroketal 6 showed moderate to high regio- and stereoselectivity (10 – 100% de), that originated from regioselective formation of the oxocarbenium ionic species and the subsequent stereoselective hydride attack. In these cases, the coordination site of the Lewis acid was controlled by the steric interaction of the methyl group on C(α) with the Lewis acid. To the resultant oxocarbenium ion, stereoelectronically-favored axial hydride attack occurred with high stereoselection. The reduction of benzyloxyspiroketal 7 also exhibited good selectivity (62 – 100% de) while the outcome was opposite to those of 5 and 6. Such a dramatic change could be attributed to the bidentate chelation of the Lewis acid to both the benzyl ether and the neighboring acetal oxygens. The whole procedure, the thermodynamic spiroketalization and the subsequent reductive ring opening, could be regarded as a remote stereochemical control using the spiroketal templates.