Carbenoid displacement

A special method involves the carbenoid displacement reaction with diazocarbonyl compounds75, which is also used in an intramolecular fashion76. 

The carbenoid displacement reaction (see Section 1.4.5.2.1.4.) of the optically active acetoxy sulfide derivative 19 (or the corresponding methoxymethyl ether) with diazomalonate in the presence of a catalytic amount of rhodium acetate in refluxing benzene affords the tram-alkylation productl22. 

A slightly different approach to 850 takes advantage of an intermolecular carbenoid displacement and intramolecular alkylation to construct the pyrrolizidine ring (Scheme 128) [193,194]. The precursor is prepared from 875 (R==OH — OEt —> SPh) and, for convenience, the protecting acyl group is replaced with a MOM group, which gives 876 as an 8 1 mixture of trans and cis phenyl sulfides. The major ra 5-sulfide is subjected to a carbenoid dis-... 

The alkaloid ( + )-retronecine (883, Scheme 129) is structurally similar to ( + )-heliotridine (850), with the exception that the stereocenter at C-7 is of opposite configuration. The basic approach to its synthesis involves a carbenoid displacement similar to that in the previous scheme. The acetyl protecting group of the common intermediate 875 (R=SPh) is changed to a TBS group, and the benzoate is converted to pivalate. Carbenoid displacement with dibenzyl a-diazomalonate in the presence of rhodium acetate gives 879. Reductive desulfurization... 

An intermolecular carbenoid reaction followed by intramolecular displacement of acetate gives the clavulanic acid derivative (112) in one step from 4-acetoxyazetidin-2-one (91) (80CC1257). Carbene-induced reactions of penicillins and cephalosporins have been reviewed (75S547, 78T1731). 

The procedures for the synthesis of cyclopropane derivatives, especially by the carbenoid route - " 49,55-57 Q, (ijg yjjjjg jQute (l.S-displacement), - have been amply reviewed. Equally well reviewed are the reactions of cyclopropanes and their use in synthetic methodologyFor the preparation of the more common cyclopropane derivatives, the use of suitably functionalized cyclopropyl building blocks that are commercially available would be recommended. 

JOC5347). When p-benzoquinone is used as a dipolarophile, the initially formed cycloadducts from 683 (R = MeO, R = H) can be treated with acetic anhydride to give compound 687, which contains the basic core dibenzo[a,d]cyclohepten-5,10-imine skeleton found in MK-801. However, it should be emphasized that cyclization of the Rh-carbenoids does not occur with weakly basic imines such as pyridines and the aromatic isoxazole system. For the synthesis of a nitrogen-bridged system, Padwa and coworkers have recently reported the successful employment of a domino transformation with displacement of a carbene center, followed by a tandem... 

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