Strategies for Seven-Membered Ring Synthesis

CYCLOADDITION STRATEGIES FOR SEVEN-MEMBERED RING SYNTHESIS 

The most common three-carbon unit utilized is the oxyallyl cation. Noyori pioneered the use of iron-promoted generation of oxyallyl cations and provided impressive examples of their reactions with dienes (Eq. 38).  

Hoffmann also made pioneering contributions to the field of [4+3] cycloadditions. These contributions include the development of processes for both intra- and intermolecular cycloadditions and of a variety of methods for generating allyl cations (Eq. 39).  

Harmata studied several variants of the intramolecular [4+3] cycloaddition including the cycloaddition of a furan tethered to an oxyallyl cation (Eq. 40), the latter generated through a clever TiCl4 initiated heterolysis of an allylic sulfone. This approach very nicely circumvents problems of performing intramolecular [4+3] cycloadditions based on a,a -dibromoketones as the allyl cation sources. 

Trimethylenemethane (TMM) derivatives, such as n-allyl palladium species, are much used in [3+2] cycloadditions. Trost showed that the methodology can also be applied in certain cases to [4+3] cycloadditions (Eq. 41).  

---

V. CYCLOADDITION STRATEGIES FOR SEVEN-MEMBERED RING SYNTHESIS... 

---