Hetero-5 + 2-cycloaddition/Claisen rearrangement

A diastereoselective tandem intramolecular hetero-5 + 2-cycloaddition/Claisen rearrangement of vinylic oxirane-alkyne substrates using a rhodium NHC complex to provide regiospecific and diastereoselective access to [3.1.0] bicyclic products has been reported (Scheme 153). ... 

The Rh-catalyzed intramolecular cycloaddition has proven reliable for the construction of polycyclic carbocycles and heterocycles. The cascade processes involving this reaction have also been achieved. A recent example is the cascade intramolecular hetero-[5 + 2] cycloaddition/Claisen rearrangement reaction of vinylic oxirane-aUcyne substrates 73, which uses the rhodium NHC complex as catalyst and provides atom-economical, regiospecific, and diastereoselective access to [3.1.0] bicyclic products 74 (Scheme 5.50) [48]. 

SCHEME 5 0 Rh-catalyzed cascade intramolecular hetero-[5+2] cycloaddition/Claisen rearrangement reaction. 

The [5+2] cycloaddition has been included in several domino processes. For example, Feng and Zhang have recentiy developed a novel, regiospecific, and diastereoselective domino intramolecular hetero-[5+2] cycloaddition-Claisen rearrangement of vinylic oxirane-alkyne substrates 90, which employed the rhodium Af-heterocyclic carbene complex, RhCl(/-Pr)(cod), as catalyst [95]. The process provided the corresponding [3.1.0] bicyclic products 91 in moderate to high yields (47-92%), as shown in Scheme 20.39. The authors have demonstrated the complete chirality transfer by performing the reaction with an enantiomerically enriched... 

Fig.4. Proposed hetero [2+4] cycloaddition-Claisen rearrangement mechanism...

The hetero Diels-Alder reaction has also been carried out efficiently in water. For instance, glyoxylic acid undergoes cycloadditions with various dienes [20] although the carbonyl function is almost exclusively present as its hydrate form. Other pericyclic reactions such as 1,3 dipolar [21] or [4+3] cycloadditions [Eq. (1), Table 1], and Claisen rearrangement [22] gave better results when conducted in aqueous media than in organic solvents. 

This type of complex also efficiently induced the intra- and intermolecu-lar [4+2] and intramolecular [5+2] cycloaddition reactions shown in Scheme 8.10. However, these eatalysts were not effective for the mtermolecular [5+2] cycloaddition reaction. The [(IPr)RhCl(COD)]/AgSbF6 system has several attractive features, such as mild reaction conditions (15-20 °C) combined with high yields (up to 99%) and high turnover numbers (up to 1900). This [(NHC)Rh]-based system also efficiently eatalysed the diastereoselective tandem hetero-[5+2] cyeloaddition/Claisen rearrangement reaction of vinylic oxiranes with all nes. A complete ehirality transfer in the tandem process was observed, thus providing a novel and efficient method to access [3.1.0] bicyclic targets in an enantioselective manner. ... 

While catalyst 22/23 has been known to be valuable in other C-C bond-forming strategies, for example, 1,3-dipolar cycloaddition [149], hetero-Diels-Alder reaction [150], Friedel-Crafts-type alkylation [151], double-Michael reaction [152], [2,3]-Wittig rearrangement [153], and Claisen-Schmidt condensation [154], only references are given here. 

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