Electrocyclic rearrangements catalysts

An alternative synthesis of the sulhlimine by reaction of the penicillin (383) with ethyl azidoformate resulted in the azetidinone (384) in low yield (Numata et al., 1972). This product was probably the result of an electrocyclic rearrangement of the intially formed sulfilimine (385). Conversion of 385 to a deacetoxycephem (290) was accomplished by heating in the presence of an acid catalyst. An analogous ring opening reaction was also observed on thermal treatment of the penicillin ester with dimethyl diazomalonate in the presence of cupric sulfate as catalyst. The initially formed ylide (386) proceeded to rearrange in an electrocyclic manner to afford the azetidinone (387) in good yield (Numata et al., 1972). 

More recently, several approaches to catalytic, enantioselective Nazarov cyclizations have been developed [28]. Trauner reported that substrate 266 underwent electrocyclic rearrangement, furnishing 268 in 94 % ee in the presence of the chiral Sc-PYBOX catalyst 267 (Equation 30) [134]. The enantio-discriminating step in this cyclization is believed to be protonation of the intermediate enolate that ensues from the Nazarov cyclization. Rueping has documented enantioselective cyclizations of related dienones in the presence of the chiral phosphoric acid derivative 270 (Equation 31) [135]. Chiral Bronsted acid catalysis thus effected the cyclization of 269 to give products 271 (87% ee) and 272 (95% ee). 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Propargylic arylaldoximes have been reported to be regioselectively converted, in the presence of copper catalysts, into four-membered cyclic nitrones via a tandem [2,3]-rearrangement and 4 r-electrocyclization of the iV-allenylnitrone intermediate (Scheme 105). (S)... 

The next sequence relies on four mechanistic steps without any catalyst (Scheme 5.51) [134], First, a disubstituted vinyl-aziridine 145 adds to dimethyl-acetylenedicarboxylate to form a divinyl-aziridine 146, an appropriate substrate for an aza-Cope rearrangement. Delocalization of the double bonds of the resulting azepine is supposed to promote a disrotatory electrocyclization to the azabicyclo-[3.2.0]-heptane 147, which tautomerizes to the final product 148. The initial chirality of the aziridine and of the (Z)-configurated double bond control the full overall diastereoselectivity of the process thanks to the conservation of orbital symmetry. 

Recently, alkynyl oximes have attracted attention as an intriguing substrate in catalytic skeletal rearrangements. Nakamura et al. reported that 0-propargylic oximes 88 derived from a,(3-unsaturated aldehydes in the presence of eopper catalysts afforded 2,3,4,5-tetrasubstituted pyridine oxides 89 (Scheme 27.32) [41]. The reaction proceeds via tandem copper-catalyzed 2,3-rearrangement and b-ir-electrocyclization of the resulting N-allenylnitrone intermediate 90. 

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