State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences , 345 Lingling Road, Shanghai 200032, China.
J Am Chem Soc. 2013 Dec 11;135(49):18396-405. doi: 10.1021/ja4064316. Epub 2013 Nov 26.
Anomerization, which involves cleavage and formation of the anomeric C-O bond, is of fundamental importance in the carbohydrate chemistry. Herein, the unexpected gold(I)-catalyzed anomerization of glycosyl ortho-alkynylbenzoates has been studied in detail. Especially, crossover experiments in the presence of an exogenous isochromen-4-yl gold(I) complex confirm that the anomerization proceeds via the exocleavage mechanism, involving (surprisingly) the addition of the isochromen-4-yl gold(I) complex onto a sugar oxocarbenium (or dioxolenium) and an elimination of LAu(+) from the vinyl gold(I) complex. The inhibitory effect of the exogenous isochromen-4-yl gold(I) complex when in stoichiometric amount on the anomerization has guided us to disclose an isochromen-4-yl gem-gold(I) complex, which is inactive in catalysis but in equilibrium with the monogold(I) complex and the LAu(+) catalyst. The proposed key intermediate in the anomerization, a transient glycosyloxypyrylium species, is successfully trapped via a cycloaddition reaction with n-butyl vinyl ether as a dienophile. SN2-like substitution of the initially formed glycosyloxypyrylium intermediate has then been achieved to a large extent via charging with acceptors in an excess amount to lead to the corresponding glycosides in a stereoselective manner.
差向异构化涉及糖苷的 C-O 键的断裂和形成，在碳水化合物化学中具有重要意义。在此，详细研究了金（I）催化的糖基邻炔基苯甲酸酯的意外差向异构化。特别是，在外源性异苯并呋喃-4-基金（I）配合物存在下的交叉实验证实，差向异构化通过外消旋机制进行，涉及（出人意料地）异苯并呋喃-4-基金（I）配合物加到糖氧鎓（或二氧戊环鎓）上，以及从乙烯基金（I）配合物中消除 LAu(+)。当以化学计量量存在的外源性异苯并呋喃-4-基金（I）配合物对差向异构化的抑制作用指导我们揭示了一种异苯并呋喃-4-基偕金（I）配合物，该配合物在催化中不活跃，但与单金（I）配合物和 LAu(+)催化剂处于平衡状态。在差向异构化中提出的关键中间体是瞬态糖基氧吡喃鎓物种，通过与作为亲二烯体的正丁基乙烯基醚的环加成反应成功捕获。最初形成的糖基氧吡喃鎓中间体的 SN2 类似取代随后通过用过量的受体充电在很大程度上实现，以立体选择性的方式得到相应的糖苷。
