No abstract

"Economy" is referred to as the thrifty and efficient use of material resources, as the principle of "minimum effort to reach a goal." More illuminating is: "the aim to portion one's forces in order to use as little as possible of them to reach a goal." Such statements certainly apply when the goal is to synthesize a complex target molecule. Redox economy then implies the use of as few redox steps as possible in the synthetic conquest of a target compound. While any sort of economy will help to streamline the effort of total synthesis, redox economy addresses a particularly weak area in present-day total synthesis. It is not enough to point out the present deficiencies, rather the purpose of this Review is to serve as a teaching tool for all practitioners of the field by giving and illustrating guidelines to increase redox economy in multistep organic synthesis.

In this tutorial review the economies of synthesis are analysed from both detailed and macroscopic perspectives, using case-studies from complex molecule synthesis. Atom, step, and redox economy are more than philosophical constructs, but rather guidelines, which enable the synthetic chemist to design and execute an efficient synthesis. Students entering the field of synthesis might find this tutorial helpful for understanding the subtle differences between these economic principles and also see real-world situations where such principles are put into practice.

Stereoselective synthesis is attracting more and more attention. Successful stereoselective syntheses require the availability of reactions of known stereochemistry which give maximum yields of the desired products. This is particularly so with reactions proceeding via cyclic transition states. Of these, the stereochemistry of [3,3]sigmatropic processes has long been known. This article now summarizes the factors determining the stereoselectivity of [2,3]sigmatropic rearrangements.Volume 18 . Number 8 August 1979Pages 563 -640 I . Introduction [3,3]Sigmatropic rearrangements (a) (Scheme 1; Claisen, Cope, oxy-Cope, hetero-Cope rearrangement)" 41 have been thoroughly studied with regard to their stereochemical course[5 and belong to the standard repertoire of stereoselective syntheses of ~lefins[~-'l or centers of chirality[* ''I. Recent outstanding applications can be seen in a juvenile hormone synthesis ['21, transformation of sugars into prostagland i n~ [ '~' , or the synthesis of tocopherol['4'. I IAmong the [2,3]sigmatropic rearrangements there is a great variety of reactions of types I and I1 [Scheme 1, eq. (b) and (c)]''~ 2'1 which frequently occur at lower temperatures than the [3,3]sigmatropic rearrangements. The five-membered cyclic transition state of [2,3]sigmatropic rearrangements shows greater conformational "flexibility" than the six-membered transition state of [3,3]sigmatropic rearrangements and should therefore be far more susceptible to the effects of stereochemical control by substituents122'. In view of this synthetic potential it was most appropriate that studies on [2,3]sigmatropic rearrangements should have been accompanied by a thorough examination of their stereochemistry which began almost simultaneo~sly[~~l. It is thus all the more surprising that the starting materials for the preparative investigation of [2,3]sigmatropic rearrangements were (purposely?) chosen in such a way that no stereochemical problems occurred. However, this meant forgoing stereochemical information.The following compilation of results should provide an insight into the stereochemistry of [2,3]sigmatropic rearrangements.Scheme I . Survey of 13.31-and [2,3jsigmatropic rearrangements. The suggested valences may be bonds to other atoms or lone pairs. Formal charges follow from the nature and the bonding situation of atoms X, Y, Z. Stereochemkal Observables['I Prof. Dr The rearrangements (b) and (c) lead to formation of a new double bond between C-1 and C-2. Given two different substituents on C-I this double bond may result in an E or 2 configuration. Hence the factors determining the configuration of the new double bond are of interest. 563The centers X and C-I of the reactants of rearrangements (b) and (c) are chiral if they each bear two different substituents. A lone pair will also be regarded as a substituent in these considerations. C-3 [rearrangements (b) [rearrangements (b)] are prochiral if the substituents on these centers are different. In the course of rearrangement, the chirality of X and c -I di...

At the brink of the 21st century, chemistry is increasingly concerned with the function that molecules fulfil as drugs, receptors, or—as ensemble of molecules—as materials. The capability of compounds to fulfil such functions cannot sufficiently be described by using only the terms composition and configuration. A decisive role is played in addition by the conformation of the molecules, which serves as the link between molecular composition and molecular function. Expressions such as “active conformation” or “competent conformation” allude to this aspect. Chemists have to develop an understanding how a flexible molecule adopts the conformation (a distinct shape) which is optimal for the function in question and how this process can be controlled. On the outset of such considerations, we may ask how nature succeeded in the process of evolution to endow flexible molecules with a preference to adopt the conformation which is optimal for the function it has to serve. In this review, I report on how we have reached a crude level of understanding of conformation design in nature with reference to the class of polyketide natural products, how we developed these insights into a conformation design of open‐chain compounds, and which applications are already in sight.