1. Fatty acid synthetase was purified to homogeneity from bakers' yeast. (2S,3R)-[3-3H~]Malic acid was prepared by incubation of fumarate with fumarase in tritiated water.3. [2-3H1 ,3-3H~]Fumarate was synthesised via a published procedure and used for the preparation of (2S,3S)-[2-3H~,3-3Hl]malate by incubation with fumarase in water.4. The above malates after being mixed with [U-14C]malate were converted into the 4-nitrophenyl hydrogen malates esterified at the C-4 carboxyl, the C-1 carboxyl and the 2-hydroxyl group being protected at intermediate stages as a 1,3-dioxolanone.5. The individual 4-nitrophenyl hydrogen malates were oxidised by zinc permanganate in carefully defined conditions to give 4-nitrophenyl hydrogen 2R-[U-14C, 2-3H1]-and 2S-[U-I4C, 2-3Hl]malonates.6. These 4-nitrophenyl hydrogen malonates were converted to the corresponding malonyl thiol esters by transesterification with coenzyme A or A'-acetylcysteamine under conditions of minimum tritium exchange and racemization.7. The malonyl thiol esters were immediately incubated with purified fatty acid synthetase and the cofactors necessary for the biosynthesis of fatty acids.8. The fatty acids, mainly palmitate and stearate, formed in these incubations were extracted and the 3H/14C ratio determined. Individual acids were recrystallised with carrier material or, after methylation, were separated by gas-liquid chromatography and the isotope ratio again determined.9. In this way it was shown that palmitate or stearate derived from the 2S-[U-14C, 2 -3 H~] malonyl thiol ester retained 51 % of the original tritium of the substrate whereas the acids derived from the 2R-[U-14C, 2-3H1]malonyl thiolester retained only 23 % of the original tritium. 2RS-[U-14C, 2-3H1] substrates gave an intermediate result.10. From a comparison of these results with those obtained previously using chiral acetate substrates it is deduced that carboxylation of acetyl-CoA catalysed by the chicken liver acetyl-CoA carboxylase occurs with retention of configuration at C-2. The results are shown to be in quantitative agreement with the theory, deduced from experiments with chiral acetates, that partial exchange of carbon-bound hydrogen occurs on the synthetase at some stage between malonate and fatty acid subsequent to the stereospecific elimination of hydrogen.11. The stereochemical course of individual reactions in the biosynthesis of fatty acids is discussed.In the foregoing paper [l] it was shown that the formation of fatty acids from chiral acetates. The fatty acid synthetase from chicken liver and bakers' discrimination between the R and S enantiomers in yeast possessed an overall stereospecificity in the these experiments was small because the small hydrogen isotope effect, operative in the acetyl-CoA carboxylase reaction, led to nearly equal proportions of tritiated malonyl-CoA species having R and Schirality. A partial, and therefore non-specific, exchange of hydrogen catalysed by the synthetase was also obAbbreviations. GLC. gas-liquid chromatography; TLC, thinlayer chrom...

analysis, isotopic labelling, and high resolution measurements were used to study the cracking patterns of methyl abscisate. Ten labelled analogues of methyl abscisate and methyl 2-trans-abscisate were available for this purpose. The principal fragmentation pathway involves ions a t mle 222, 190, 162, 134, 106, and 91, and specific labelling of each oxygen atom with oxygen-18 proved especially helpful in defining the structures of the intermediate ions. Other important fragmentations of the molecular ion are the loss of methanol, the loss of water, and the cleavage of the complete side chain to give the abundant ion at m/e 125.

Approaches to the synthesis of 4,6-diaryl-5-hydroxydibenzophosphole 5-oxides are described. 3,5,7-Trihydroxydibenzophosphole 5-oxide and several of its O-substituted derivatives were made either from 5-hydroxydibenzophosphole 5-oxide (convenient preparation described) by dinitration, reduction and tetrazotization or from 2,2'-dibromo-4,4'-dimethoxybiphenyl via lithiation and reaction with dichloromorpholinophosphine with subsequent oxidation. 3,5,7-Trihydroxydibenzophosphole 5-oxide could be selectively blocked in the 2,8-positions by methyl groups generated via a Mannich reaction. The bis-2-iodobenzyl ethers of 3,7-dihydroxy-5-methoxydibenzophosphole 5-oxide and of its 2,8-dimethyl derivative were cyclized by photolysis to form 4,6-diaryl derivatives in poor yield.

In view of the current interest in the electron-impact induced rearrangements of both trimethylsilyl derivatives and bifunctionalized cyclohexanes, the mass spectral properties of several alkoxycyclohexanol trimethylsilyl ethers and alkoxycyclohexyl trimethylsilanes have been investigated. Using deuterium labeling, the basic fragmentation modes incurred upon electron-impact of ethanol trimethylsilyl ether were elucidated. The major peaks in the spectrum of this simple ether occur also in those of the title compounds, the latter fragments having been formed by interaction of the two functional groups. Thus the initial rearrangement ion of mass 103 (e) from electron bombardment of 4-ethoxycyclohexanol trimethylsilyl ether and 2-ethoxycyclohexyl trimethylsilane corresponds to the [M -CH,] ion from ethanol trimethylsilyl ether. Its further fragmentation has been studied using high resolution and metastable ion measurements, and the scope of the interaction between the alkoxy and trimethylsilyl groups, when silicon is attached both to oxygen and to carbon, has been determined. In addition, the origin of every 'characteristic' fragment ion has been elucidated on the basis of each corresponding peak in the spectra of several derivatives labeled specifically with deuterium. Peaks derived from initial ionization of both functionalities in each compound have been observed, although the majority of ions in the spectra of both series seem to be directed by fragmentations associated with the trimethylsilyl ether moiety.