Claisen rearrangement allylic systems

Pyrimidine annulated heterocycles fused at positions 5 and 6 to uracil were synthesized via a three-step sequence starting from uracil 63 [20]. Firstly, the reaction with 3-bromocyclohexene gave the AT-allyl-vinyl core system 64 in 80% yield. Upon heating 64 in EtOH in the presence of HCl, aza-Claisen rearrangement gave rise to the C-cyclohexenyl uracil 65 in 38% yield. Final bromination ( 66) and dehydrogenation steps ( 67) allowed synthesis of the desired tricyclic fused uracil systems (Scheme 15). 

The third mechanism starts with addition of the AT-allylamine 103 to the cumulated acceptor system of an allene carbonester 108 (Acc=CHC02Me) to form an intermediate iV-allyl ammonium amide enolate 109 (allene carbonester Claisen rearrangement). The anion stabilizing group is exclusively placed... 

Organomagnesium compounds react with imines, prepared from 3-methoxy-2-naphth-aldehydes by a 1.4-addition mechanism. This reaction can be performed with high diastere-oselectivity. The method was applied for the synthesis of optically pure S-tetralones . Vinyhnagnesium bromide reacts as an acceptor with a ketone dimethyl hydrazone zincate 207, yielding a 1,1-bimetallic species, which can be reacted sequentially with two different electrophiles (equations 131 and 132) . The reaction proceeds via a metalla-aza-Claisen rearrangement, where the dimethylhydrazone anion behaves as an aza-allylic system . 

If the allylic system is substituted, several isomeric products can be formed. Assuming a chair-like transition state, the stereochemical outcome of a metallo-Claisen rearrangement is controlled by the geometries of the vinyl and allyl moieties, so that, if the vinyl part 68 is stereochemically pure, three different products, syn/anti-73 or 74, can be formed, depending on the exact nature of the active allylic part 72 (Scheme 6)38. 

The sequence with oxazole 73 also proves to be applicable to systems in which the double bond of the allylic alcohol fragment is incorporated into a heteroaromatic ring, such as in furfuryl alcohol (77a) and thiophene-2-methanol (77b).28 After hydrolysis of the oxazolone ring with water, V-benzoyl-3,3,3-trifluoro-2-(2-methyl-3-furyl)alanine (79a) and /V-benzoyl-3,3,3-trifluoro-2-(2-melhyl-3-thienyl)alanine (79b) are obtained. The 2-methylene helerocyclcs 78 originally formed in the Claisen rearrangement undergo rearomatizalion under the reaction conditions. 

The disconnection for the aromatic Claisen is to reverse the rearrangement. This is a little simpler than those we have seen so far as one C-C bond is broken 59 and one C-0 bond made. But you must remember to turn the allylic system back to front. This is easily seen if the starting material is drawn as 59a with the dotted line representing a reconnection. The rest is a normal ether disconnection. 

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