Suprafacial route

Since the hydrogen bonding orbital is a Is orbital, a thermal reaction will follow the suprafacial route if k is odd, and the antarafaciai route if k is even e.g. 

The occurrence of a 1,7-photochemical shift of H in this compound does not, of itself, establish directly that this shift proceeds via a suprafacial pathway. The relatively rigid cyclic structure of (47) must, however, rule out the possibility of the shift having proceeded via the antarafacial route. 

Carbenes are much more reactive toward carbon-carbon double bonds than toward single bonds. Without doubt the most useful feature of a elimination is that it provides a practical route to cyclopropanes and cyclopropenes by [2 + 1 ] cycloaddition of carbenes to double or triple bonds. These additions are stereospecific suprafacial additions if they involve singlet carbenes, but can give mixtures with triplet carbenes ... 

In an excited state (i.e. photochemically) the HOMO is now ij 4 and it is antisymmet-rical. Therefore, [1,5]-suprafacial migration of H is no longer possible i.e. it is symmetry forbidden. The photochemical reaction will follow the antarafacial route, which is... 

Evans rather general route to y-substituted allyl alcohols is depicted in a specific example, which demonstrates the stereochemical outcome of the suprafacial rearrangements in cyclic systems (Scheme 14).65... 

There are two possible spatial routes the hydrogen atom can select in the transformation XXIX XXX in one (suprafacial), the hydrogen moves across the face of the plane described by the olefin chain (XXXI) and in the other (antarafacial), the hydrogen passes through the plane to the opposite face of the olefin plane (XXXII). 

The many recent publications affirm the importance of this reaction in modem synthesis. Tanaka and Takemoto s synthesis of the marine metabolite halicholactone uses a sulfoxide route to the allylic alcohol 115. Notice the suprafacial [2,3]-sigmatropic rearrangement and that the OH group ends up in the middle of the diene. Attempted cyclopropanation of 115 gave a poor yield of a mixture of products.21... 

Rearrangement of 2H-pyrroles to the thermodynamically more stable 1//-isomers can be envisaged as occuring via a thermally allowed suprafacial [l,5]-sigmatropic shift by a C-2 substituent, either to nitrogen (route A,... 

Since its discovery over sixty years ago [1] the Diels-Alder reaction has lost none of its attraction. [2, 3] It enables, in a one-step inter- or intramolecular reaction, the rapid preparation of cyclic compounds having a six-membered ring. During the course of the [4 -I- 2] cycloaddition four new stereocenters can be introduced directly, and their stereo-control is a topic of major interest in modem synthetic chemistry. [4-6] In addition, in intermolecular reactions, the relative positions of the reaction partners (regiochemistry) must be taken into account. If a concerted reaction is assumed, both a cis addition (suprafacial mode) and a preferred endo orientation (Alder rules) can be expected. But how can the absolute configuration of the desired product be controlled There are three basic possibilities the use of a chirally modified diene, a chirally modified dienophile, or a chiral catalyst. Although the first successes resulted from the attractive, hut difficult, catalytic route, [4b, 7] the majority of the investigators are concerned with the stoichiometric... 

The lanthanide catalyzed hetero Diels Alder reaction is a promissing route to many heterocycles where various stereoselectivities could be achieved by structural changes of the catalyst (57,58). The activation of the aldehyde by coordination can be envisaged as the key step of the process, which is followed by a suprafacial attack of the diene with an endo approach of the aldehyde hydrogen atom. 

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