Ring compounds bridgehead position

PN and Parent Exo PX) have unsubstituted norbornenyl rings and differ from each other only in the stereochemistry of their ring fusion. These compounds have been elaborated by the incorporation of either a phenyl (tf>) or a carbomethoxy (C) group in the bridgehead (B) or vinyl (V) positions. Thus, the notation VN represents PN with a phenyl substituent at a vinyl position, while CBX represents PX with a carbomethoxy substituent at the bridgehead position. The isomerization and polymerization chemistry of these nine compounds (PN, PX, 4>BN, (jiVN, (jiVX, CBN, CBX, CVN, CVX) are the main concerns of our work. 

Deuterium atom was neatly incorporated at the bridgehead position C-l in ketone 464, the only compound in which the cyclohexane ring is locked in a boat conformation. Examination of molecular models indicates that the cyclohexanone ring can easily adopt a boat form in 460 and 461. It appears to be more difficult with ketone 462 and almost impossible with ketone 463. 

BREDT S RULE A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. 

Compound (a) is stable. Although the double bond is at a bridgehead, it is not a bridged bicyclic system. The trans double bond is in a 10-membered ring. Compound (b) is a Bredt s rule violation and is not stable. The largest ring contains six carbon atoms, and the trans double bond cannot be stable in this bridgehead position. 

The differences in conformation of penicillin, A - and ALcephems and cephams have already been studied by different methods [39,267-270]. The steric position of the 3-carboxy group in penicillins is closer to that in A -cephems than to that in A -cephems. On the other hand, the bridgehead nitrogen atom of penicillins and A -cephalosporins is definitely pyramidal, in contrast to its planar structure in A -cephems and cephams Table 8.29) [269]. /3-lactams fused with 7 or 8 membered rings (compounds... 

An extension of this reaction type was the intramolecular addition of a carbanion to the ring, which provided a rare type of bicyclic compound (91) with phosphorus at a bridgehead position (Equation (16)) <870M586>. 

---