Studies on diterpenoids and related model compounds

• Main Author: Pannell, L. K.
• Language: en
• Published: University of Canterbury. Chemistry 2013
• Online Access: http://hdl.handle.net/10092/7988

The acid-catalysed dehydration of' the [FORMULA] (38) gave initially [FORMULA] . Under more vigorous conditions, these olefins reacted to give an equilibrium mixture consisting largely of the rearranged [FORMULA] and [FORMULA]. On long-term reaction, the five olefins: were converted into the [FORMULA] (46). The solvolysis of the saturated tosylate (66) in formic acid gave products: (67,68) resulting from elimination of the tosylate group ,and subsequent formic acid addition to the olefins formed the monocyclic formates (69,70,71). The analogus reaction of' the unsaturated tosylate (73) gave similar products (74,78,79,80) but, in addition, gave two epimeric bicyclic formates (75a, 75b) which arise via reprotonation of' the diene (71b). The reaction of this tosylate (75) with trifluoroacetic acid was investigated. In this case, the predominant products were trifluoroacetates (83) and (85). In addition, some direct cyclisation products were also isolated (81,82). The reactions of the ketones (57;n=0,1,2) with boron trifluoride- dichloromethane were investigated. The ketone (57,n=1) gave two rapid cyclisation to two bicyclic alcohols ( 89) and (90).The ketone (57,n=0) gave slow reversible cyclisation to the bicyclic alcohols (100,101). However, the product of long-term reaction was the conjugated ketone (99). A dimeric product and some other minor components were also identified. The ketone (57,n=2) rearranged to give ketone (108), which underwent cyclisation to give products (111,112,113,114). No products of direct cyclisation of ketone (57,n=2) were detected. The reactions of the ethyleneketals of ketones (57;n=0,1) were investigated. For the ketal (n=0), deketalisation occurred more rapidly than cyclisation. However, for ketal (94;n=1) cyclisation occurred to give two bicyclic hydroxy-ethers (95) and (96). The reactions of the bicyclic alcohols (89,90,95,96,100,101 ) with boron trifluoride-benzene were investigated. For systems (n=1), the bridgehead oxygen function was replaced by a phenyl group and the products isolated were (91) and (92). However, for systems (n=0), an additional pair (102,103) of phenyl-substituted products were isolated. The relevance of these cyclisation studies to the biosynthetic pathway to tetracyclic diterpenoids is discussed, and a possible modification to the Wenkert scheme proposed.