Ring Expansion Claisen Rearrangement

The aza-[2,3]-Wittig rearrangement [47] and the related aza-[3,3]-Claisen rearrangement [48] of vinylaziridines are elegant examples of expansion of the aziridine ring in a stereocontrolled fashion (Scheme 38). 

Some other ring expansions involving the intramolecular amino Claisen rearrangement of vinylarylaziridine [ 123], the Diels-Alder reaction of indoles with acetylene derivative [124-127] and the dibromocarbene insertion into quinoline enol ethers [ 128] have been used to prepare 1-benzazepines. On the other hand, treatment of 3-chloro-3-phenyl-l,2,3,4,5,6-hexahydro-l-benz-azocin-2-ones with piperidine causes a ring contraction to give 2-phenyl-2-(l-piperidinylcarbonyl)-2,3,4,5-tetrahydro-l//-l-bcnzazepines in an excellent yield [23]. 

An elegant ring expansion methodology has been developed by Paquette [699] (Figure 3.41). This synthetic sequence is based on the double methylenation of an a-(acyloxy)aldehyde to give an allyl vinyl ether, which undergoes (reductive) Claisen rearrangement when treated with trialkylalanes. 

Byproducts 9 arise from internalization of the double bond of 7, caused by traces of the methylenation reagent, followed by [3,3] sigmatropic rearrangement. A chairlike transition state was indicated by the reactions of 7c and 7d180. The rearrangement of 7e is the key step in the total synthesis of ( + )-precapnelladiene179 I80. For the total synthesis of ( + )-7,8-epoxy-2-basmen-6-one by a similar reaction sequence, see ref 336. For the enantiospecific synthesis of the tricyclic nucleus of acetoxycrenulide by Claisen ring expansion see ref 337. 

Zwitterionic [3,3]-sigmatropic rearrangements. The zwitterionic aza-Claisen rearrangement is useful for ring expansion. It is subject to 1,2-asymmetric induction. ... 

With a variety of catalyzed and uncatalyzed reaction conditions for the Michael addition and aza-Claisen rearrangement, ring expansion was explored by reacting the five- and six-membered amines (31) and (32) with dimethyl acetylenedicarboxylate (DMAD), as the Michael acceptor <94JA579> (Equations (5) and (6)). The cyclic enamines are rather sensitive to hydrolysis during... 

Conceptually, this rearrangement is related to the Bellus-Claisen Rearrangement, in terms of ring expansion. This reaction is also related to the regular Claisen Rearrangement and Cope Rearrangement. 

The Claisen rearrangement of cyclic allyl vinyl ethers provides corresponding ring expansion products. This method is widely used for total syntheses. 

White has reported the use of the Claisen rearrangement to implement a ring expansion strategy from the ketene acetal 63 which was generated from cyclic carbonate 62 via Petasis conditions yielding the lactone 64. ... 

---