The development of new catalytic asymmetric reactions can lead to exciting new strategies for organic synthesis. This article describes the synthetic utility of the combined COH activation͞Cope rearrangement, achieved by dirhodium tetraprolinate-catalyzed reaction of vinyldiazoacetates with compounds containing allylic COH bonds. The transformation is highly diastereoselective and enantioselective. The product distribution, however, is highly substrate dependent, the major side products being either direct COH activation or cyclopropanation.T he field of chiral catalysis has experienced explosive growth over the last two decades (1, 2). By now, many of the classic reactions of organic synthesis can be achieved in a highly asymmetric manner. One of the new frontiers for asymmetric catalysis is the discovery of new catalytic asymmetric methods that do not have an established achiral counterpart (3). For some time we have been exploring the chemistry of donor͞acceptor-substituted rhodium carbenoids (2) as a possible source of these new catalytic asymmetric transformations (Eq. 1) (4). Because of the presence of the donor group (typically vinyl or aryl), 2 are much more stabilized than the conventional carbenoids, which contain only electron acceptor groups (typically ester, keto, phosphonate, sulfonate, cyano or nitro) (5, 6). Consequently, 2 undergo a range of transformations with unprecedented regioselectivity and stereoselectivity (4).Our studies into the chemistry of donor͞acceptor-substituted carbenoids have already led to a number of exciting methods for catalytic asymmetric synthesis. The dirhodium tetraprolinate complex Rh 2 (S-DOSP) 4 [S-DOSP is S-(N-dodecylbenzenesulfonyl)prolinate; see Eq. 2] is an exceptional chiral catalyst for these carbenoids, resulting in generally high levels of asymmetric induction (7). These methods include cyclopropanation of alkenes (Eq. 2) (8), [3 ϩ 4] cycloaddition between vinylcarbenoids and dienes (Eq. 3) (9), [3 ϩ 2] cycloaddition between vinylcarbenoids and vinyl ethers (Eq. 4) (10), and intermolecular COH activation (Eq. 5) (11). A common feature of all of these transformations is the opportunity for the stereoselective generation of multiple stereocenters in a single step.Intermolecular COH activation is a very exciting reaction because it represents an additional strategic reaction for organic synthesis. Complementary carbenoid versions of many of the classic synthetic reactions of organic chemistry have been achieved. -Hydroxy esters, usually derived from an aldol reaction, are stereoselectively formed by COH activation ␣ to oxygen (Eq. 6) (12).