The
conformation of the side chain of 5,6,7,8-tetrahydrobiopterint (6) in 0.5 M DCl/D2O
is predominantly quasi-equatorial (deduced from 3J (13C4a, 1H6)
1.1 HZ), and is the same as that of the methyl group in 2-methyl-1,2,3,4-tetrahydroquinoxaline and in 2-amino-6-methyl-5,6,7,8-tetrahydropteridin-4(3H)-one
in the same solvent. Because (-)-(2S)-2-methyl-1,2,3,4-tetrahydroquinoxaline(4)
and (-)-(6S)-2-amino-6-methyl-5,6,7,8-tetrahydropteridin-4(3H)-one
(5) have the same conformation and negative c.d. spectra (O 248 nm and
263 nm respectively) as does the natural 5,6,7,8, tetrahydrobiopterin (O minimum
at 265 nm) in 0.1 M hydrochloric acid, then the
absolute conformations of the tetrahydropyrazine
rings and the absolute configurations
at the chiral centres C2, C6,
and C6 of compounds (4), (5) and (6) respectively are the same. Hence the
absolute configuration at C6 in natural 5,6,7,8
tetrahydrobiopterin is R.� A convenient synthesis of biopterin
on a gram scale is described.
The chemistry of indoles. XVI. A convenient synthesis of substituted indoles carrying a hydroxy group, a halogeno group, or a carbon side chain at the 4-position via 4-indolediazonium salts and a total synthesis of (.+-.)-6,7-secoagroclavine.