E. E. Van Tamelen

Cyclization studies with 14,15-oxidogeranylgeranyl acetate

Abstract As a model for more elaborate biogenetic-type syntheses of naturally occurring 3-hydroxylated tetra- and pentacyclic terpenoids, the acid-induced cyclization of 14, 15-oxidogeranylgeranyl acetate (1) was studied. Through stannic chloride catalysis, the epoxide 1 was transformed into the tricyclic diol monoacetate 12, along with monocyclic ketone 29, acyclic chlorohydrin 32, and bridged 1,4-oxide 33. The structure and stereochemistry assigned to the tricycle 12 were confirmed by conversion to a reduction product, monoacetate 26, which was indistinguishable on spectral grounds from a substance obtained by an independent synthesis starting from manool (17).

The biogenetic-type total synthesis of ajmaline

Abstract Chemical steps 14→15→20→21→22→23a→23b→24a→24b→25→29→30→42→43→44→45→46→1 constitute the first biogenetic-type total synthesis of the indole alkaloid ajmaline.

Cyclization studies with (±)-10,11-oxidofarnesyl acetate, methyl farnesate, and methyl (±)-10,11-oxidofarnesate

Abstract As models for lanosterol biosynthesis and for more elaborate biogenetic-type total synthesis in the di- and triterpenoid series, the nonenzymic cyclization of methyl farnesate, methyl (±)-10,11-oxidofarnesate and (±)-10,11-oxidofarnesyl acetate was studied. In addition to the 10,11-halohydrin, N -bromosuccinimide/water was observed to convert methyl farnesate to bromobicycles 40 and 41 . Acid-catalyzed cyclization of (±)-10,11-oxido farnesate and the corresponding 2,3-cis isomer produced inter alia 3-hydroxy octalins 20a and 21a . Similarly, methyl (±)-10,11-oxidofarnesate was transformed to inter alia the hydroxylated bicyclic esters 34 and 35 . Mechanistic and other aspects of these reactions are taken up.

Biogenetic-type total synthesis of (±)-farnesiferol A and (±)-farnesiferol C

Abstract Biogenetic-type cyclization of trans,trans -umbelliprenin oxide 4 generates the natural occurring farnesiferol C ( dl ) ( 3 ), in addition to various other unnatural transformation products. Similar cyclization of the cis,trans counterpart ( 34 ) of 4 affords inter alia farnesiferol A ( dl ) ( 1 ). Bioorganic and physical organic aspects of these changes are discussed.

Cyclization of N[2-(3-indolyl)acetyl]-5-aminovaleraldehydes and N[2-(3-indolyl)ethyl]-5-aminovaleraldehydes

Abstract Various model cyclizations related to the organic or biological synthesis of diverse indole alkaloids, are described and discussed, including: 18 → 20, 23 → 25, 28 → 31 and 29 → 35, 37, 38 and 39 (as aldehydes).

The biogenetic-type total synthesis of methyl adirubine

Abstract The first total synthesis of methyl adirubine ( 11a ), representing the 5-carboxy- Corynanthe indole alkaloid group, has been completed. The biogenetic-type sequence, comprising new steps 27 → 28 → 29 → 16 → 17 → 33 → 18 → 34 → 35-IV → methyl adirubine, also defines the stereochemistry of the latter as 11b .

Biogenetic-type synthesis of hydroxylated tricyclic diterpenes in the pimarane class

The biogenetic-type synthesis of various diterpenoids (in the racemic form) possessing the pimarane backbone was achieved through nonenzymic cyclization of the oxide of methyl geranylgeranyl carbonate (46). Treatment of oxide 46 with BF3 · Et2O in CH3NO2 effected formation of pimaradienol 36, isopimaradienol 39, and the naturally occurring tricycle 40. Acid treatment of either 39 or 40 led to an equilibrium mixture which includes isomer 41, also of natural origin. Side-chain oxidation of dienol 40 afforded araucarol (16), a third plant product. Other substances formed in the cyclization of oxide 46 are described, and a mechanistic interpretation of the overall reaction course is presented.

The rapid separation and isolation of mono- and oligo-nucleotides by high speed liquid chromatography: An ion exchange, reversed-phase system

Summary A new chromatographic material has been developed which has a combination of reversed-phase and ion exchange characteristics. In conjunction with high speed liquid chromatography, it provides a convenient method for the analysis and purification of nucleotides and their derivatives.

Reaction of unsymmetrical dienophiles with a key dienoid intermediate for aphidicolin synthesis

Abstract The Diels-Alder reactions of diene (6) with α-chloroacrylyl chloride and nitroethylene afford adducts (11) and (12), as proven by X-ray crystallographic analysis of a key transformation product (10), supplemented by appropriate correlation experiments. Formation of these adducts, rather than the less sterically congested isomers (7) and (9), can be rationalized using frontier molecular orbital theory.

The role of substrate structure in the initiation of enzymic cyclization of squalene 2,3-oxide. Stereochemistry of homosterol formation from 1-methylsqualene 2,3-oxide

Squalene 2,3-oxide lanosterol cyclase selectively converts the 1′-cis-methyl isomer as a component in a mixture of cis- and trans-1-methylsqualene 2,3-oxides into 4α-ethyl-4β,14α-dimethylcholesta-8,24-dien-3β-ol.