Stereocontrolled Synthesis of Highly Functionalized Quaternary Carbon Centers: A Route to α‐Substituted Serines

Abstract

The development of efficient stereoselective methods for the formation of CÀC bonds involving the construction of a quaternary carbon centre has attracted much attention. [1] These processes become more useful and challenging if additional neighboring stereogenic centers and polar functionalities are also formed stereoselectively. In this context, our research group is interested in developing methodologies for the construction of substructures bearing a densely functionalized quaternary center; like those found in asubstituted threonines 1 or serines 2, [2] as well as in more complex natural products of biological relevance such as the proteasome inhibitor lactacystin (3) [3] or the immunosuppressant myriocin (4). [4] We envisaged that quaternary amino acids such as 1 or 4 could arise from a homoallylic alcohol I which, in turn, could be obtained by the stereoselective addition of a substituted allylic organoborane II to an aldehyde (Scheme 1). Despite the extensive use of various terpene-and tartrate-based chiral allylborane as well as crotylborane reagents, [5] the required stereoselective addition of g,g-disubstituted allylboranes to aldehydes has been much less explored. [6] Examples of the stereoselective creation of quaternary carbon centers by addition of g-heteroatom allylboranes such as II are still scarce. [7] The limited use of such g,g-disubstituted allylboranes in organic synthesis is probably due to the difficulty involved in their stereoselective preparation. To overcome this drawback, we anticipated that II could be prepared in a straightforward manner by hydroboration of allene III.Our initial proposal for III was allene 5, since it can be easily obtained in two steps from but-2-yn-1,4-diol (Scheme 2). [8] We reasoned that its hydroboration at the less hindered face of the terminal double bond would generate an unsymmetrical Z allylborane, which could be stereoselectively added to an aldehyde to generate the amino diols 6 that are protected at the quaternary center. To our delight, we found that the treatment of 5 with dicyclohexylborane in Scheme 1. Retrosynthetic analysis of I.Scheme 2. Synthesis of 6 a by addition of 5 to isobutyraldehyde. Cy = cyclohexyl, Ts = 4-toluenesulfonyl.