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and ( + ) -( Z ) +A + F + C -(-)-(E) are consistent with the solvent effect on kb. Structure D either as a transition state or intermediate might provide less charge separation than F, and could be used to explain why k , > kb. A rapidly reversible D e E stage is equally attractive. In effect, inclusion of such stages in epimerization at cyanoacetate provides a path for a conducted tour of C+ of benzyl from C-to 0-, to inverted C-, to product inverted at the cyanoacetate center, and resembles ionic conducted tour mechanisms suggested in other connections.6 Intervention of intermediates G, H, and J in the epimerization at the cyanoacetate center in D M F also might explain k , > kb in this medium. No such structures are available for epimerization at the benzyl center. Racemization rates appear equally sensitive to solvent character, and probably the same intermediates are involved as in the two epimerization reactions. For example, (+)-(Z) + A + B or Cenantiomer of A + (-)-(Z) are the simplest processes.(6) (a) D.peratures and the more polar solvent favor larger spreads in rates of the three reactions. In DMF, in going from 100 to 126O, k,/kb decreased by 3.3 for a 26" temperature rise. Extrapolation to 175' gives only a factor of 2.7, not far from benzene at 175" (1.6). Possibly temperature differences are mainly responsible for changes in k,/kb, not solvent polarity or nucleophilicity.Mechanisms of the three reactions might involve intermediates A-J. Of these only E is not an ion pair.5 Mechanism ( + ) -( Z ) + E+ (+)-(E) is incompatible with the dramatic and similar response of k, and kb to solvent polarity. Epimerization at benzyl (kb) cannot involve E as an intermediate. Mechanismsdefinitely are compatible with the solvent effects and nonaccumulation of E only if k-1 >> kl > kz.This scheme would make fortuitous the similar response of k , and kb to solvent polarity. An analogous mechanism for the I $ E + D or G stages was found not to occur in methanol,2a although k, and k (methanolysis) are not far from one another in value. These facts make such a scheme highly improbable. Mechanisms that involve G, H, and J are not possible in benzene, and yet epimerization occurs. A B E F G H D Mechanisms (+)-(Z) -A -B --t (+)-(E), (+)-( Z ) -A -D -B -(+)-(E), and (+)-(Z) + A ---t D E -B + (+)-(E), with the first stage rate determining, are consistent with the solvent effect on k,. Mechanisms ( 5 ) Intervention of ketene acetal in (+)-(Z)-I -(+)++I would nicely correlate this particular epimerization reaction with the wellknown aldehydo or acylcyclopropane * dihydrofuran rearrangement Sir:The use of o-nitrobenzyl derivatives as photosensitive blocking reagents for amino and carboxyl functions in peptides has beenIn this communication we describe some new photosensitive blocking groups, and conditions required for achieving photoremoval in quantitative yields.Amino acid derivatives, in which the amino function was blocked with photosensitive protecting groups of two kinds, namely, 6-nitroveratryloxycarbonyl (NVOC) and 2-n...

Orbital symmetry controls in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction.Chemistry remains an experimental science. The theory of chemical bonding leaves much to be desired.

1. Einleitung gen, die es iiberstand, und die aus ihm abgeleiteten und bestatigten Voraussagen sind eindrucksvoll. Im folgenden geben wir eine ausfiiliche Beschreibung unserer Vorstellungen, zeigen einige Entwicklungen auf, die in " Unter allen Moglichkeiten, die es heute zur theoretischen Behandlung der chemischen Bindung gibt, darf das Molekiilorbital-Verfahren als dasjenige gelten, das sich in der Hand des organischen Chemikers als am fruchtbarsten erwiesen hat und das den Bediirfnissen des Organikers am besten entspricht. Dennoch ist dieses Verfahren mit nur wenigen Ausnahmen vor allem zum Studium statischer Molekiileigenschaftenim Grund-und Anregungszustandverwendet worden, und nur selten hat man versucht, seine Aussagekraft an reagierenden Systemen zu erproben. 1965 haben wir in drei vorlaufigen Mitteilungen 11-31 die Grundlagen fur die theoretische Behandlung aller Synchronrealctionen gelegt. Wie sich diese Ideen entwickelten, ist anderswo nachzulesen [41. Ihr Prinzip besteht darin, d& solche Reaktionen glatt verlaufen, wenn zwischen den Orbitalsymmetrie-Eigenschaften der Reaktanden und Produkte Ubereinstimmung herrscht, und daB ihr Ablauf mit Schwierigkeiten verbunden ist, wenn diese Ubereinstimmung fehltoder kiirzer gesagt, &J bei Synchronreaktionen die Orbitalsymmetrie erhalten bleibt. Dieses Prinzip hat weithin Beachtung gefunden; seine Anwendungen, die Priifun-

Figure 3. Interconversion of cyclopropyl cation and allyl cation.As to the photochemical transformations, the first excited state of either radical should also isomerize by the conrotatory mode, but in the second excited state, which lies very close above the first, disrotatory transformations are to be expected.We have thought it worthwhile to supplement the discussion of Woodward and Hoffman in this way because orbital and state correlation diagrams enable one to follow a reaction along its entire course and draw qualitative conclusions without necessarily engaging in numerical computations.(2) E. W. A., at present on leave from the Case Institute, Cleveland, Ohio, wishes to thank the National Institute of Neurological Diseases and Blindness for a Fellowship.