The dec a rb o xyla tiv e elimination of (32) to (2 3 ) has been shown*1** to proceed in an a n ti-fash ion , and t h e re fo re it might be con certed , as shown in Scheme 1.7 . H ow e ve r, although the
16 -
s t e r e o c h e m is t r y o f th e elimination is a n t i , w h e t h e r th e h y d r o x y l g r o u p is p h o s p h o r y la t e d p r i o r t o , o r concom ita ntly w it h , t h e d e c a r b o x y l a t i o n , has not bee n e s t a b lis h e d .
T h e f o llo w in g s t e p is an en z ym e-m ed iated , r e v e r s i b l e iso m erisa tion o f i s o p e n t e n y l p y r o p h o s p h a t e ( 2 3 ) in to d im e t h y la lly l p y r o p h o s p h a t e (2 4 ) ( c f . Scheme 1 . 1 ) . In th is p r o c e s s a methyl g r o u p is g e n e r a t e d in a t r a n s r e la t io n s h ip t o t h e m e th y le n e ph o s p h a te m o iety.
T h e r e i s , as sh ow n in Scheme 1.8, an a n t i - r e la t io n s h ip b e t w e e n th e p r o t o n a d d e d to t h e d o u b le bond (on t h e S i - f a c e ) and th a t re m o v e d from t h e 2 -p o sitio n ( H - R e ) .
T h e i s o p r e n y l b u ild in g b lo c k s a re r e a d y t o be a s s e m b le d , and t h is is d o n e b y en zym e s called p r e n y l t r a n s f e r a s e s . I n i t i a l l y , g e r a n y l p y r o p h o s p h a t e ( 2 5 ) is g e n e r a t e d ( c f . Scheme 1 . 2 ) b y th e c o u p lin g o f is o p e n t e n y l p y r o p h o s p h a t e ( 2 3 ) and d im e t h y la lly l p y r o p h o s p h a t e ( 2 4 ) . T h e s t e r e o c h e m is t r y o f t h is r e a c tio n has bee n e lu c id a te d b y C o r n f o r t h ^ ’ d u r i n g his s t u d ie s on t h e b i o s y n t h e s is o f s t e r o i d s . T h e re a c tio n is a formal a lk y la t io n in w hic h th e p y r o p h o s p h a t e g r o u p o f d im e t h y la lly l p y r o p h o s p h a t e ( 2 4 ) is r e p la c e d with i n v e r s i o n o f c o n f i g u r a t i o n , b y is o p e n t e n y l p y r o p h o s p h a t e (2 3 ) ( c f . Scheme 1 . 9 ) . T h e r e a c tio n is a s y n p r o c e s s , and 70-71
it has been proposed that it is stepwise. Poulterand his c o -w o rk e rs have studied the reactions of f a r n e s y l pyrophosphate syn th eta se , and in partic ula r, the l ' - 4 condensation between isopentenyl p y r o phosphate ( 2 3 ) and dimethylallyl pyrophosphate (2 4 ) or g e r a n y l py rophosp hate ( 2 5 ) . T h e y establis hed that the condensation reactions are electrophilic and have three main phasest a ) an elimination
o f the 1' p y r o p h o s p h a t e , g e n e r a t i n g a ca tion ic s p e c ie s that is b ) a t ta ck ed b y th e d o u b le bond o f i s o p e n t e n y l p y r o p h o s p h a t e (2 3 ) g e n e r a t i n g a n ew ca tion ic s p e c ie s from w hich c ) a p r o t o n is elim inated. T h e in t e r m e d ia t e ca tion ic s p e c ie s d o e s not lo s e stereochem ica l i n t e g r i t y b ec au se it is p r e s e n t as a t i g h t l y bou nd ion p air at th e e n z y m e 's a c t i v e s i t e . T h e ion p a ir is th e n a t ta c k e d b y the d o u b le bond o f is o p e n t e n y l p y r o p h o s p h a t e ( 2 3 ) . Be th a t as it may, t h e r e m a rk a b le s t e r e o c o n t r o l e x e r t e d d u r i n g th is p r o c e s s has been e x p lo ite d f o r s t e r e o s p e c i f i c C - C bond fo rm atio n in t h e e n z y m e - a id e d s y n t h e s is o f about 30 h om olo gu es o f f a r n e s y l p y r o p h o s p h a t e ^ . T h e p r o c e s s d e s c r i b e d a b o v e is th e b i o s y n t h e s i s o f ( 2 E , 6 E ) - fa r n e s y l p y r o p h o s p h a t e ( 1 7 ) . Many p r e n y l t r a n s f e r a s e s g e n e r a t e 67 t h is s t e r e o c h e m is t r y a b o u t the d o u b le b o n d s , as is the c a s e o f two g e r a n y l t r a n s f e r a s e s isola ted from R ic in u s communis^ . F o r t h e b io s y n t h e s is o f c y c l i c s e s q u i t e r p e n e s , h o w e v e r , iso m er iz a tio n about the 2-d o u b le b o n d must o c c u r b e f o r e c y c lis a t io n can t a k e p l a c e .
- 18 -
It has bee n shown t h a t a c e l l - f r e e e n z y m ic s y s t e m o f A n d r o g r a p h i s
3 14
paniculata t r a n s fo r m s ( E , E ) - l l , l - H 2 S l2 ,1 3 - C2l f a r n e s y l p y r o ph o s p h a te ( 3 3 ) into ( Z ) - ^ - b is a b o le n e ( 3 4 ) w ith o u t lo s s o f tritiu m ( c f . Scheme 1 . 1 0 ) ^ .
19 C u paran e T r i c o t h e c a n c L a u r in t e r is a n e
òto b£
Lau rane S e s q u ic a ra n e T h u jo p s a n e B e rg a m o ta n e . CK-Santalane /3-Santalane A c o r ia n e A c o r a n e C e d r a n e A n is ata n e Scheme 1.11 S t r u c t u r a l r e la t io n s h ip o f Some S e s q u i t e r p e n e s20 -
An i n t e r e s t i n g point is that the same cell f r e e system
3 3 t r a n s f o r m s ( E , E ) - [ 1 , 1 - ^ j f a r n e s o l into (_Z , E ) — 11 — H jf a r n e s o l w it h lo ss o f the p r o - S h y d r o g e n and r e t e n t io n o f t h e p r o - R h y d r o g e n at t h e 1 -p o s it io n . T h ese o b s e r v a t io n s in d ic a t e t h e p r e s e n c e o f tw o i n d e p e n d e n t e n z y m e systems, an ( E , E ) - f a r n e s y l p y r o p h o s p h a t e 75 66
iso m era se and an ( E , E ) - f a r n e s y l p y r o p h o s p h a t e is o m e r a s e -c y c la s e T h e d e t a ile d mechanism o f the c y c lis a t io n h as not bee n e l u c i d a t e d , a lt h o u g h t h e r e is a la rg e amount o f in fo r m a tio n about la b elled 76 78 com pou nds th a t a r e in corporated in to s e s q u i t e r p e n e skeleton s ’
”y - B is a b o le n e ( 3 4 ) is one o f t h e most w i d e l y d is t r ib u t e d s e s q u i t e r p e n e s . In Scheme 1.1 1, the s t r u c t u r a l r e la t io n s h ip betw een s e v e r a l s k e le t a l t y p e s o f s e s q u it e r p e n o id s is i l l u s t r a t e d . In vie w o f t h is r e l a t i o n s h i p , a c e n t r a l role has bee n p r o p o s e d f o r the immediate p r e c u r s o r t o b is a b o le n e . T h u s , th e b i s a b o l y l cation ( 3 5 ) , v ia sig m a tro p ic h y d r o g e n shifts and W a g n e r - M e e r w e in t y p e ske letal
75
rea rra n g e m en ts, gives rise to a g re a t n um ber of the sesquiterpenoid stru c tu ral t y p e s . Attractive though this proposal may be, confirmatory e vid ence is lacking.
Biosynthesis of the turmerones p re su m ably follows the same course as that of 7~bisabolene ( 3 4 ) . H o w e v e r , when the cyclisation o f fa rn e sy l pyrophosphate g e n e r a te s the bisabolyl cation ( 3 5 ) ,
21
X
22
a 11 , 3 ] - p r o t o t r o p i c sh ift g e n e r a te s an a lly lic e n d o c y c lic cation (3 6) w h ic h can lose a proto n e i t h e r at t h e 6- p o s it io n o f t h e r i n g , o r at the 7 -m eth y l g r o u p to g e n e r a t e the c o r r e s p o n d i n g s e s q u it e r p e n e s ( 3 7 ) and ( 3 8 ) . Both h a v e been r e p o r t e d to be c o n s titu e n ts
81 o f t h e oil o f tu rm eric
S tu dies o f the oil b y GC/MS in dicate t h e p r e s e n c e o f su bsta n ces 82
w i t h m/z 202 and 204 (s e e F i g . 1.1 3 ) . It is p r o p o s e d that t h e s e p r e c u r s o r s a re acted upon b y a m o n o o x y g e n a s e , which g e n e r a t e s an alcohol. S e s q u ite r p e n o id alcohols r e la t e d t o th e t u rm e ro n es h a v e also been r e p o r t e d as c o n s tit u e n t s o f the o il, e ve n i f t h e ir
83 p r e c i s e s t r u c t u r e was not e lu c id a te d
T h is m o n o - o x y g e n a s e may be akin to tho se r e s p o n s ib le fo r w -o x id a t io n o f f a t t y a c id s , and we p r e d i c t that th e s o u r c e of o x y g e n is bound atm ospheric d i o x y g e n . T h e s e in te rm e d ia te alcohols m a y then b e o x id is e d b y a d e h y d r o g e n a s e to g e n e r a t e Q!- and j3~tu rm e ro n es which a re f u r t h e r o x id is e d t o g e n e r a t e a r - t u r m e r o n e . T h e o x id a tio n o f