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Dedicated to Professor Raymond U. Lemieux on the occasion of his 65th birthdayThis account examines double asymmetric induction from theoretical and practical viewpoints. In the context of four major organic reactions-the aldol, Diels-Alder, catalytic hydrogenation, and epoxidation-it is shown that a double asymmetric induction can be analyzed in terms of the single asymmetric reactions of each of the two chiral reactants. A rule which qualitatively relates the results of these single asymmetric reactions with the outcome of the double asymmetric reaction is proposed. A powerful new strategy based on this rule for the predictable creation of new chiral centers is discussed and the use of this strategy for the synthesis of sugars and macrolides is presented.

Professor Raymond U. Lemieux zum 65. Geburtstag gewidmetBei der doppelten Stereodifferenzierung (doppelt asymmetrischen Synthese) wird ein chirales Substrat rnit einem chiralen Reagens umgesetzt. Dieser Aufsatz befaBt sich rnit der doppelten Stereodifferenzierung unter theoretischen und praktischen Gesichtspunkten. An vier bedeutenden organisch-chemischen Reaktionen -der Aldol-und der Diels-Alder-Reaktion, der katalytischen Hydrierung und der Epoxidation -wird gezeigt, daB dieses Phanomen als Funktion der einfach asymmetrischen Reaktionen eines jeden der beiden chiralen Reaktanten rnit einem achiralen Partner analysiert werden kann. Es wird eine Regel vorgeschlagen, die das Ergebnis dieser einfach asymmetrischen Reaktionen und den stereochemischen Verlauf der doppelten Stereodifferenzierung qualitativ verkniipft. Mit einer leistungsfahigen neuen Strategie, die auf dieser Regel aufbaut, kbnnen Chiralitatszentren in vorhersagbarer Weise eingefiihrt werden: die Strategie hat sich bereits bei der Synthese von Zuckern und Macroliden bewahrt. EinfuhrungDer Begriff der asymmetrischen Synthese ist in der Organischen Chemie wohlbekannt.

Asymmetric Diels-Alder Reaction: Design of Chiral Dienophiles Summary: An asymmetric Diels-Alder reaction between cyclopentadiene and a new chiral dienophile is described. Excellent diastereofacial selectivity can now be achieved even in the absence of a catalyst. The stereochemical result of this reaction has been rationalized on the basis of a rigid hydrogen-bonded cisoid conformation of the ketol-type dienophile.

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Diels-Alder Reactions of -Oxy-o -xylylenes1Summary: Treatment of benzocyclobutenols or their acetates with n-butyllithium at 0 °C or less can generate the corresponding -oxy-o-xylylenes, which can then undergo Diels-Alder reactions. Sir: The formation of the reactive but elusive o-xylylenes (5,6-bis(methylene)-l,3-cyclohexadienes) followed by their Diels-Alder reactions constitutes a powerful synthetic sequence for the construction of many cyclic natural products.1 However, the typical temperatures (>25 °C) employed for this sequence have frequently been dictated by

or substituted acetic acid unit and can, in principle, be effected with tert-butyl, methyl, and trimethylsilyl ester Supplementary Material Available: Detailed spectroscopic data (IR and NMR) for compounds 10a,d, 11,12a,b, and 14a,b (2 pages). Ordering information is given on any current masthead page.(21) Under these reaction conditions trans olefinic -keto esters 14a and 15a did not undergo ester hydrolysis or isomerization to the cis isomers. Treatment of pure trans-15a with HBF4 for 24 h afforded only recovered ester. The crude products were a mixture of -pyrones 20 and 21, trans -keto esters 14a and 15a, and uncyclized cis -keto acids 14b and 15b, respectively. Exposure of the initial 1,2-addition product to HBF4 for shorter periods of time (2-4 h) afforded larger quantities of -keto acid 14b. Attempted separation of these product mixtures by chromatography on silica gel afforded pure a-pyrones 20 and 21 in yields that were unaffected by the quantity of -keto acids 14b and 15b present in the original mixture. Similarly, -keto acid 11 underwent cyclization to -pyrone 17 when subjected to medium-pressure liquid chromatography (MPLC) on silica gel.(22) When the reaction of 8 with ieri-butyl lithioacetate was quenched with 1.2 equiv of 10% HBF4 14a was isolated as a 5:1 Z/E mixture of stereoisomers. The structures of these stereoisomers were assigned on the basis of proton NMR chemical shifts for the methylene protons (CDC13; E 3.01, Z 3.53). A methyl substituent cis to the carbonyl functionality in /3-methyl-a,/S-unsaturated carbonyl compounds resonates downfield in the NMR spectrum relative to the trans methyl substituent:

Purpose – The purpose of this paper is to present a review of empirical studies on principal leadership in Singapore. It seeks to provide a general picture of Singapore principals’ leadership qualities, styles, and roles. Design/methodology/approach – This is a systematic review of empirical studies, using a “bounded” approach with a focus on the main findings of the reviewed studies. In all, 36 studies were selected for the interview. The findings of these studies were open coded, synthesized, and clustered into different themes. Findings – The review revealed several qualities, characteristics, styles, and enacted roles of Singapore principals. While there are similarities between Singapore principals and principals elsewhere in the world, the review brought out some features unique to Singapore principals. Originality/value – This review contributes to the growing literature in comparative research on principals’ leadership and their enacted roles, and concurrently functions as a guide for further research on school leadership in Singapore.