Conformational Analysis of Tetrahydroprotoberberines by Carbon-13 NMR Spectroacopy

Abstract

g n a b l e b y comparison of t h e chemical s h i f t of C(6) in carbon-13 nmr s p e c t r o s c o p y .I t i s p o s s i b l e , f u r t h e r m o r e , to d i stinguish the position of s u b s t i t u e n t s in 9.10and 10, 11-substituted tetrahydroprotoberberines by the d i f f e r e n c e in chemical s h i f t of ~( 8 ) .Diben~o[a,~]quinolizidine i s the skeleton o f tetrahydroprotoberberine a l k a l o i d s .If the B and C r i n g s of diben~o[a,~]quinolizidine take half-chair conformations.i t e x i s t s in the equilibrium of one trans (1) and t w conformations (2 and 3).but mainly as t h e thermodynamically more stabletrans-quinoli~idine.~ X-Ray analys e s of the hydrobromides of capaurine a n d i s o c a p a u r u n i n e , and capaurimine p bromobenzoate revealer!that they e x i s t as the cis-quinolizidine form (2) in the c r y s t a l l i n e state.'It was c o n s i d e r e d that a n e n e r g e t i c a l l y unfavorable nonbonded i n t e r a c t i o n of the C -1 substituent t o the C -13 hydrogens destabilized the t r a n s form.S u c h a n unfavorable i n t e r a c t i o n s t i l l remains in the o t h e r &form ( 3 ) ,