Cyclobutanones from ketene complexes

Similar to the thermal reaction of ketenes with alkenes, the photolysis of alkoxycarbenes 18 in the presence of (electron-rich) olefins 19 leads to cyclobutanones (Scheme 6) [9]. In these reactions the sterically more strained [2 + 2] cycloadducts of type rac-17 are generally formed with good regio- and diastereoselectivity. Starting from complexes of type 21, the intramolecular version of this reaction affords bicyclic products of type rac-20 (Scheme 7) [9]. 

Irradiation of alkoxycarbene complexes in the presence of aUcenes and carbon monoxide produces cyclobutanones. A variety of inter- and intramolecular [2 + 2]cycloadditions have been reported. The regioselectivity is comparable with those obtained in reactions of ketenes generated from carboxylic acid derivatives. Cyclobutanones can be obtained with a high degree of diastereoselectivity upon reaction of alkoxy carbenes with chiral A-vinyloxazolidinones. For example, photolysis of (19) in the presence of (20) gives cyclobutanone (21) (Scheme 31). In addition to aUcoxycarbenes, carbenes having a thioether or pyrrole substituent can also be employed. Related intramolecular cycloadditions of y,5-unsaturated chromimn carbenes afford bicyclo[2.1. IJhexanones (Scheme 32). 

Fused and spirocyclic ring systems are present in many natural products and drug molecules. An approach to these systems was developed by [2 + 2] cycloaddition between 2,3-dihydrofurans and ketenes generated from ali-cyclic acid chlorides (14TL7240).This approach allows the construction of complex-fused, spirocyclic cyclobutanone frameworks in simple operations, even on a large scale (20 mol). 

---