Stereoselective Synthesis of Branched Cyclopentitols by Titanium(III)-Promoted Reductive Cyclization of 4-Oxiranylaldehydes and 4-Oxiranyl Ketones Derived from Hexoses

Abstract

Titanocene chloride efficiently promotes the intramolecular reductive cross coupling of highly functionalized 4-oxiranylaldehydes and 4-oxiranyl ketones derived from readily available hexoses affording branched cyclopentitols with good stereoselectivity.Epoxides are a highly valuable and versatile group of compounds widely used in organic synthesis due to their varied reactivity and their straightforward preparation from readily available alkene or carbonyl precursors, generally with high control over their relative and absolute stereochemistry. The strained three-membered oxirane ring is prone to ring-opening reactions with nucleophiles and low-valent-metal reagents. Both types of ring-opening reaction complement each other in terms of polarity and regioselectivity. Thus, while nucleophiles preferentially attack the less substituted carbon of the oxirane ring, single electron transfer from low-valent-metal reagents 1 produce the most substituted b-metal oxide C-radical regioselectively, which subsequently adds to electrophilic radical acceptors.Due to our longstanding interest in the exploration of new strategies for the stereoselective synthesis of highly functionalized cyclitols starting from readily available carbohydrate precursors, 2 we decided to explore the disconnection strategy shown in Scheme 1 for the direct preparation of branched cyclitols of structure I from 4-oxiranylcarbonyl substrates II. A number of possible mechanistic scenarios could be contemplated for the cyclization step, each involving a different generic reactive intermediate A-D. Purportedly, we could envisage the C-C bond-forming step as proceeding through a radical (a, d) or a carbanionic pathway (b, c). In the first case, the required radical intermediates A or D could originate via single electron transfer (SET) reduction of the epoxide or the carbonyl group, respectively. In the ionic alternative, generic carbanionic intermediates B or C could be formed from II either by two sequential SET reductions via radicals A and D or, directly, by a bielectronic process involving the oxidative addition of a low-valent-metal reagent to the epoxide or the carbonyl group. Examples of C-C bond-forming reactions following each of these mechanistic pathways have been described in the literature, 3 with the only exception of path d, which has no precedent.Our group has recently described the first successful reductive cross coupling of epoxides with aldehydes and ketones 4 promoted by samarium diiodide, for which we proposed an epoxide-derived C-radical of type A as the key reactive intermediate in the C-C bond-forming step. The reaction took place intermolecularly and provided a new access to C-glycosides from 1,2-anhydrohexoses in a stereoselective way, which complemented other approaches previously described. With the intention of exploring an intramolecular version of this reaction that could give access to polyoxygenated branched carbocycles, we decided to synthesize appropriate substrates from readily available hexoses.Using D-glucose as the ...

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