Ring Opening and Closing Reactions

Finally, the formation of platinum(II) ylide complexes from halomethyl derivatives is worthy of note [equation (7)]. Palladium analogs are more 

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Figure 1.22 The Fischer glycoside synthesis, illustrated by reaction of methanol and glucose. The ring opening and closing reactions are shown as stereospecific, by analogy with transglycosylations, whereas the displacements at Cl of the pyranosides is nonstereopsecific.

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

Based on enantioselective epoxidation and subsequent ring opening and closing, the so-called Achmatowicz reaction was developed. This is an organocatalytic one-pot cascade for the annulation of a,(J-unsaturated aldehydes, hydrogen peroxide, p-carbonyl compounds and NBS, which furnish optically active 3-pyrones. Other chiral heterocycles were also assembled by organocatalytic cascade reactions using diaiylprolinol silyl ethers as catalysts. ... 

It was argued that 9-(3 -cyclopropenyl)-cw" -cyclononatetraene was formed by a disrotatory ring opening and closed to the two stereoisomeric cis-(3 -cyclopropenyl)indenes, one of which gave the product via an intramolecular Diels-Alder reaction. 

In this chapter, we will see electrocyclic reactions in which rings open and close, cycloaddition reactions in which two partners come together to make a new cyclic compound, and sigmatropic shifts in which one part of a molecule flies about coming to rest at one specific position and no other. Specificity is the hallmark of all these reactions atoms move as if in lock step in one direction or another, or move from one place to one specific new place, but no other. The chemical world struggled mightily to understand these mysteriously stereospecific reactions, and, thanks to the work by Woodward, Hoffmann, and several others, finally figured it out in a marvelously simple way. This chapter will show you some wonderful chemistry. 

In 1970 ring-opening and ring-closing reactions of 2,2-dialkyl-1,3,2-dioxa- and -1,3,2-oxathia-stannolanes with carbon disulfide and phenyl isothiocyanate involving nucleophilic attack at the tin atom were discovered <70CC235>. Since these reactions are described as proceeding via a cyclic transition state they will be considered in Section 4.24.2,5.10. 

Figure F. Opening and closing of chelate ring in transition state plot of energy vs. reaction coordinate for ZnAc or a one-bonded ox.

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