Stereochemical double bond systems

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

The only difference between the two reactions evidently consists in the shift of the double bond in respect of the methyl group, from a, b, to b, c. For between a and b, through stereochemical reasons, no double bond can exist, since, in accordance with Bredt s rule, none of the C-atoms which are common to both rings of a bicyclic system of the camphene type can take part in an unsaturated linkage. 

The chemistry of the bicyclo[2.2.1]heptane ring system, with and without double bonds in the two-carbon bridges, includes many well-known and interesting features which illustrate important stereochemical aspects of some organic reaction mechanisms. Among derivatives of... 

Medium-sized and large ring systems often show complicated conformational interconversions involving pseudorotations in one or even more conformational families. This makes stereochemical assignments in diastereomers rather difficult. Thus, very few systematic studies have been published. The situation is improved if such rings are embedded in polycyclic systems, or if they contain double bonds, which leads to restricted conformational mobility. An example is the differentiation of diastereomeric 2,3-dihydro-lf/-benzo[6]azepines 1 on the basis of y-gauche effects and on d(13C) and 3/H H values640. 

The most successful examples of stereochemical control in electrophilic heteroatom cyclizations are those in which the substitution pattern constrains the substrate so that the two diastereofaces of the tt-system are significantly different. The most straightforward prediction of stereochemistry involves incorporating both the ir-system and the directing chiral center into a ring such that rotation about the vinylic bond that attaches the nucleophile to the double bond is highly restricted. Comparison of equations (1) and (2) illustrates this difference. For this reason, in the sections on cyclizations to form five- and six-membered rings, examples with constrained C=C—C bonds will be discussed separately. 

The stereochemical implication for the ejection of a leaving group in the B-position of a carbonyl group which yields an a,B-unsaturated system has been treated elsewhere (see p. 233). It may be pointed out here that double bond formation through the opening of a cyclopropane ring should also take place following the same stereoelectronic principle. One example of such a reaction is the transformation of B,r-cyclopropyl-s-hydroxyketone 299 which is smoothly converted into the dienone 300 (85) under acid conditions. 

A major drawback of alkene metathesis is lack of control over the stereochemistry of the newly formed double bond. For unstrained systems, E/Z ratios are virtually unpredictable. Alkyne metathesis, on the other hand, can always be combined with subsequent Lindlar hydrogenation, thereby giving access to stereochemically pure 2-olefins. In 1998, Ftirstner and Seidel were the first to report a ring-closing alkyne metathesis [7]. Under high-dilution conditions (0.02 m) and reduced pressure (20 mbar, removal of 2-butyne, solvent 1,2,4-trichlorobenzene (b.p. 214 °C)) the Schrock catalyst was applied to assemble macrocydic... 

---