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The activation parameters [∆G (200 K), ∆H , and ∆S ] for compounds, together with those for related compounds previously reported in the literature, show that conjugating the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a subsituents (e. g. aryl, CN, CO 2 R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas number of their 3,7-and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF 3 , SO 2 Ar) retard the reaction. spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c, d, e, h, j, k, Substituents in the 3,7 positons have little effect or are rateretarding. The presence of 1,5-tri-and -tetramethylene and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5hϪ8h. Careful attention has been given to the bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former. measurement of temperatures. The data for these

A general method is developed using *H and 13C NMR chemical shifts to determine unambiguously the predominant tautomeric form of many known aryl cyclic amidines and guanidines, 2-aminoimidazoles, 2-imino(amino)thiazines, and related tautomeric systems. In the case of 2-aryliminopyrrolidines, evidence for geometrical isomerism was found in both and 13C NMR experiments. These results support the conclusion that, in all these potentially tautomeric systems under the present studies, the predominant tautomer is in the imino form [ArN=C(NHR)R'] rather than the amino form [ArNHC(=NR)R'].

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Whereas lithium phenolate in dioxolane coexists as a mixture of comparable amounts of a hexamer and tetramer, together with minor amounts of less aggregated species, in THF the predominant species is the tetramer. Two crystalline forms of lithium phenolate have been obtained from THF and THF/hexane solutions, and X-ray crystallographic studies show that they consist of very similar hexagonal prismatic hexamers of lithium phenolate with one THF molecule bound to each lithium cation. The 7Li nuclear quadrupole coupling constant (QCC = e2Qqzz/h = 67 f 1 kHz) and the asymmetry parameter (7 = 0.77 f 0.05) have been obtained from line shape analysis of the powder spectrum of one of the crystalline samples. Proton decoupling was used to remove 1H/7Li dipoledipole interactions, and samples diluted with 6Li were employed to estimate the contribution of the 7Li/7Li interaction to the line shape.These data give a value (73 kHz) for the quadrupole splitting constant [QSC = (1 + ~2/3)112(eZQqzz/h)] in agreement with that (72 f 10 kHz) determined for hexameric lithium phenolate in dioxolane from spin-lattice relaxation data. Thevalues of QSC for the tetrameric species in THF and dioxolane are 46 f 0.5 and 59 f 6 kHz, respectively. CP-MAS 13C NMR spectroscopy is of limited value for characterizing solid samples of the aggregates.The reactions of organic lithium compounds with electrophiles in weakly polar aprotic solvents are known to be influenced by thestateof aggregation of thelithiumreagent. Thosecompounds in which lithium is bound to oxygen most commonly exist as dimers and/or tetramers. More recently, however, the formation of hexamers has been established and, in one case at least, a hexamer has been shown to be the primary reactant.' In this paper, we present a full characterization of the hexameric lithium phenolate (LiOPh*SOlV)6 in the solid state (Solv = THF) and in solution (Solv = dioxolane), which we regard as archetypal hexamers. The study includes X-ray structures as well as characterization of the species in both the solid state and solution by 13C and 'Li NMR spectroscopy.Several hexameric lithium enolates have previously been characterized in the crystalline state.2 The unsolvated hexamer of lithium enolate derived from pinacolone generated with LDA in heptane has a hexagonal prismatic structure.2a.b A similar structure has been found for the lithium enolate of ethyl N,Ndiethylglycinate, in which the five-membered chelate rings stabilize the hexamer.2c Lithium N-isopropylbenzamide forms the same type of hexameric crystals in THF/hexane when 1 equiv of TMEDA is added."r This aggregate, which contains three THF molecules but no TMEDA, is also stabilized by chelation. Hexamers of (hexamethy1enimido)lithium have tricyclic laddertype structures.3Our earlier studies of solution species revealed the existence of hexameric lithium phenolates in the weakly polar aprotic solvent dioxolane. For example, in dioxolane at low temperatures, both lithium phenolate and its 3,s-dimethyl homologue coexist as the Hodgson, S. M.;...

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3829between I3C CP/MAS spectra of amorphous starches and V-type amylose complexes provides evidence for the presence of amylose/lipid inclusion complexes in starch granules. adjacent to a high-energy glycosidic linkage. Apart from these signals, chemical shift ranges are similar to those of V-amyloses.Solution state C-l and C-4 chemical shifts for cu-cyclodextrin are closely matched if averages of solid-state shifts are taken which ignore the sites adjacent to the high-energy linkage. This suggests that a-cyclodextrin might adopt a more expanded conformation in solution compared to the solid state. The marked similarity Registry No. a-Cyclodextrin, 10016-20-3; a-cyclodextrin hydrate, 5 121 1-5 1-9; 8-cyclodextrin, 7585-39-9; 8-cyclodextrin hydrate, 68 168-23-0; CY-( 1-+4)glucan, 9051-96-1; amylose, 9005-82-7; starch, 9005-25-8. Abstract:The structures of lithium 4-fluoro-, 4-chloro, 2-and 4-bromo-, 4-(trifluoromethy1)-, 4-methoxy-, 2-methyl-, 2-ethyl-, 2-n-propyl-, 2-isopropyl-, 2-tert-butyl-, 2-(methoxymethyl), 3,5-dimethyl-, 3,5-dimethyl-4-methoxy-, 4-chloro-3,5-dimethyI-, 3,5-diethyl-, and 3,5-dimethoxyphenolates in solution in weakly polar, aprotic solvents such as pyridine, tetrahydrofuran, dimethoxyethane, 1,3-dioxolane, diethyl ether, 2,6-lutidine, and triethylamine have been established by 13C NMR spectroscopy. Para substituents influence the equilibrium between dimer and tetramer through their effect on the basicity of the anion. @Alkyl substituents promote dimer formation through steric effects in the order of their steric bulk. The 2-methoxymethyl group stabilizes the tetramer. Dimers are favored relative to tetramers by a combination of high Lewis basicity and low steric demand of the solvent. A number of lithium phenolates in 1,3-dioxolane exist as hexamers at low temperatures. The following pairs of values for AH (kcal mol-I) and A S (cal mol-]

Structures have been established for the polyenes, phytoene, phytofluene, <-carotene and neurosporene by spectral studies and synthesis. The stereochemistry of these polyenes is discussed.IT is now generally accepted2'3 that phytoene, phytofluene, <-carotene, and neurosporene are intermediates in the biosynthesis of carotenoids, rather than artefacts or by-products of this process.4~~ Each is believed to be converted into the next member of the series 1