RCM-ROM-CM

In contrast to 52 (Scheme 12), diastereomer 55 (Scheme 13), because of its more exposed and highly reactive strained olefin, undergoes rapid polymerization in the presence of 4a. The less reactive Ru complex 56 [23] can however be used under an atmosphere of ethylene to effect a tandem ROM/CM to generate 57. The resulting triene can be induced to undergo Mo-catalyzed ARCM (5 mol % 4a) to afford optically pure 58, the AROM/RCM product that would be obtained from 55. 

In the final synthesis a silicon tethered olefin was used as a protecting group that facilitates a RCM-ROM-RCM sequence. An alternative strategy was to carry out ROM-RCM with TBS protected alcohol (Scheme 20) and subsequent CM, but this strategy was low yielding. 

One does not immediately associate a reaction which generates sp1 carbon centres with asymmetric inductive capability, however the development of non-racemic catalysts such as 40, 41 and 42 (Fig. 6) has allowed the efficient synthesis of optically active alkenes via the kinetic resolution (KR) of dienes and the desymmetrisation of meso-alkenes via either RCM or ROM-CM. For a short review of asymmetric metathesis see Ref. [85]. 

The cascade alkene metathesis processes described above result from the combination of ROM and RCM. The cascade alkene metathesis reaction involving ROM, RCM and CM reported in Scheme 18 leads to [n.3.0]bicycles in a stereo-controlled manner [40]. The reaction combines ring opening of... 

The enantioselective synthesis of azabicyclic y-lactams starting from 2-azanorbornenones after treatment of a catalytic amount of RuCl2(PCy3)2 (= CHPh) in the presence of ethylene or allyl acetate proceeds also via ring rearrangement—alkene metathesis (ROM-CM-RCM) [41] (Scheme 19). If n = 0 or 3, no RCM occurs and a cyclic dialkenyl compound is formed by cascade ROM-CM reactions. 

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