André S. Dreiding

Application of the α-alkynone cyclization : synthesis of rac-modhephene

Abstract Using the thermal α-alkynone cyclization 8 → 9 as the key step, modhephene 1 , a sesquiterpene with a [3.3.3]-propellane skeleton was synthesized.

Generation and comparison of space-filling molecular models

Abstract A computer system is presented for the bitwise encoding of three dimensional space-filling molecular models. The system permits the computation of molecular volumes and surface areas. By superposition of two space-filling models a degree of similarity between molecules can be defined.

The conversion of betanidin and betanin to neobetanindin derivatives

Abstract Details are given for the conversion of betanin, the pigment of the red beet, and its aglucone, betanidin, to a number of derivatives of neobetanidin trimethyl ester. The conversion occurred, presumably by air-oxidation, when the basic conjugated system was deprotonated under the influence of diazomethane, pyridine and acetic anhydride, amines or sodium acetate. The neobetanidin-esters turned out to be 14, 15-dehydrobetanidin derivatives and were the key for the structure elucidation of the betacyanins and betaxanthins. Their constitutions could be derived essentially by comparison of the NMR spectrum of the 5,6-di-O-acetyl derivative with those of two degradation products from betanidin, namely N-acetyl-5,6-diacetoxy-2,3-dihydroindole-2-carboxylic-acid methyl ester (now called triacetylcylodopa methyl ester) and 4-methyl-pyridine-2,6-dicarboxylic-acid methyl ester. A separate proof for the vinylene group connecting the two heterocycles was available through its reduction in a Pd catalyzed reaction at the expense of a dehydrogenation of the dihydroindole system. The same type of disproportionation reaction could also be applied ot N-styrylindolin, which was converted to N-(β-phenylethyl) indole. The neobetanidin structures contain a “1,7-diazaheptamethin” system. This new expression, which is explained briefly, allows a rationalization of the absorption spectra (color), the halochromism (about 100 nm), the pKa-values (1·6 to 2·4) and certain NMR signals of the neobetanidin derivatives.

The constitution of vertinolide, a new derivative of tetronic acid, produced by Verticillium intertextum☆

Abstract From the culture of Verticillium intertextrum a new tetronic acid derivative, vertinolide, has been isolated as the main constituent of the chloroform extract. Structure 1a was established for vertinolide by X-ray diffraction analysis. Vertinolide (1a) was also transformed to an O-methyl-(1b), an O-acetyl-(1c), a tetrahydro- (1d) and a tetrahydro-O-methyl-derivative (4). The spectroscopic properties of 1a, of its derivatives 1a, 1c, 1d and 4 as well as of three model compounds are compared and discussed.

Chemical Review and Evolutionary Significance of the Betalains

The betalains, which include the red-violet betacyanins and the yellow betaxanthins (Fig. 10.1), represent a structurally and biosynthetically distinct class of naturally-occurring pigments. These pigments are found in flowering plants only in members of certain families of the order Caryophyllales (Centrospermae) (Table 10.1), though betaxanthins have also been shown to occur in the fly agaric, Amanita muscaria (Dopp et al. 1982). Both betacyanins and betaxanthins possess a dihydropyridine moiety which is attached via a vinyl group to another nitrogenous group. In the betacyanins the latter is a glyeosyl-hydroxylated dihydroindole ring; in the betaxanthins it is any one of several amino acids or amines. The basic chromophore of the betalain pigments is the 1,7-diazaheptamethinium system, as discussed by Mabry et al. (1967) and Mabry and Dreiding (1968). The different spectral properties of the betacyanins and betaxanthins are due to the extension of this conjugated system through the dihydroindole moiety in the betacyanins. The term “betalains” was introduced by Mabry and Dreiding (1968) to emphasize the common biogenetic and structural features of these two groups, which clearly distinguish this class of pigments from the more common group of plant pigments, the anthocyanins.

Preparation of 2-vinylcyclobutanones and their conversion to cyclopentenones

Abstract Methyl- and ethyl-vinylketene 3 were added to several simple olefins 1 to afford alkylated 2-vinylcyclobutanones 4 , which in turn rearranged to cyclopentenones 6 (via dienones 5A , B ) or 7 (via 1,2-acyl migration) under acid catalysis. Evidence is presented for a cyclopropyl-carbinyl intermediate 8 .

The synthesis of (±)-isoptychanolide by application of the α-alkynone cyclisation

Abstract The first synthesis of racemic isoptychanolide (2) , a stereoisomer of natural ptychanolide (1) , which differs in the configuration of the oxirane function, is reported. The key steps were an α-alkinone cyclisation ( 5 → 4 gas phase flow thermolysis at 620°), a stereoselective exo allylation of the enolate ( F ) derived upon desoxygenation and reduction of the enol ether 10 , and elaboration of the two substituents at C (3) of the resulting 11 to afford the enol lactone 3 . Epoxidation of the double bond of 3 took place from the side of C(2) (and not of C(4)), so that isoptychanolide ( 2 ) was formed. The configurations of all stereocentres in 2 were determined by an X-ray analysis.

On the structure of [η4-2,5-bis(trimethylsilyl)cyclopentadienone](η5-cyclopentadienyl)cobalt

X-ray diffraction study has been carried out on [η 4 -bis(trimethylsilyl)cyclopentadienone](η 5 -cyclopentadienyl)cobalt (I), the first example of a (η 4 -cyclopentadienone)(η 5 -cyclopentadienyl)cobalt complex with only two substituents on the cyclopentadienone ring. Complex I crystallizes in space group P 2 1 / c with four molecules in the unit cell and lattice parameters a 6.5259(5), b 12.2378(9), c 21.8397(15) A and β 90.442(7)°. Extended Huckel MO calculations are discussed for a simpler model, namely the parent cyclopentadienone complex (III).

Synthesis of a stereoisomer of ptychanolide

Abstract A stereoisomer of ptychanolide 1 was synthesized by a method which includes an α-alkynone cyclization.

Effioent Synthesis of Terminal Bis(Trimethylsilyl)-α,ω-Diynes

Abstract Bis-trimethytsitylation of the di-MgX-salts of terminal α,ω-diynes (1, n = 2-6 and 14) proceeds with high yields under high dilution conditions.