Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0739 884

(equation 238)318. Thermal oxy-Cope rearrangements were used to form the diterpene (equation 239)319 and oxygen-bridged sesquiterpene (equation 240)320 frameworks.

OH

MeO

(587)

(238)

O

MeO

The oxy-Cope rearrangement can be carried out using catalysis by mercury trifluoroacetate (equation 241)321 as well as an antibody catalysis322. Reaction of two equivalents of 1-lithio-3,4-dihydronaphthalene with acenaphthenequinone at 0 – 20 °C affords a derivative of tricyclo[4.3.0.05,9]nonane 589 by double oxy-Cope rearrangement (equation 242)323. Another example of a little known double oxy-Cope rearrangement is the reaction of tricarbonyl chromium complex 590, which undergoes a sequential transformation consisting of the double addition of vinyl lithium derivatives to the keto groups and subsequent double oxy-Cope rearrangement under very mild conditions ( 78 °C) (equation 243)324.

HgX

R1, R2 = Me, R1R2 = (CH2)4, (CH2)10 (35−75%)

X = OCOCF3

E. Hetero-Cope Rearrangements

In principle, Cope-type rearrangements can occur in any 1,5-diene system consisting of six carbon and/or heteroatoms (equation 244). However, despite the apparent variety of potential possibilities, few examples of hetero-Cope rearrangements are known up to now. It should be noted that the structures depicted in equation 244 which can generally contain up to six heteroatoms are no longer real dienes. Nevertheless, we will briefly

consider the principal types of such systems as well as their transformations (for reviews, see Reference 325).

(242)

OH

H

H

HO

(589)

1. Claisen and related rearrangements

Among the hetero-Cope processes the Claisen rearrangement is the most known. It was discovered in 1912326 as the first in a series of related [3,3]-sigmatropic isomerizations such as the Cope rearrangement. The synthetic significance of this reaction is obvious even by the numerous reviews about Claisen rearrangement (a list of 25 surveys since 1940 till 1979 is given in Reference 327; see also reviews 208 and 326). In principle, the Claisen rearrangement can be generalized by equation 245, i.e. it is a rearrangement of aliphatic allyl vinyl ethers to ,υ-unsaturated carbonyl compounds. It has been well established that a Claisen rearrangement proceeds through a cyclic chair transition state as an intramolecular concerted [3,3]-sigmatropic isomerization. The influence of substituents on the Claisen rearrangement328,329 as well as some stereochemical aspects such as face selection have been studied recently330. Various synthetic methods such as tandem thermal Claisen – Cope rearrangements of coumarate derivatives 591 (equation 246)331,332 were developed on the basis of Claisen rearrangement.

O

R

+

Li

O

Cr(CO)3

(243)

(244)

X Z X Z

Y Y

O O

(245)

Further, the elegant biogenetic-like method for constructing steroid systems by a cascade of cyclizations deserves special attention. A simple, highly stereoselective version of the Claisen rearrangement leading to trans-trisubstituted olefinic bonds was discovered in 1970 by Johnson and colleagues333. This method is based on heating an allylic alcohol 592 with excess ethyl orthoacetate in the presence of a trace of weak acid (e.g. propionic acid). The dialkoxycarbenium cations evidently formed under such conditions react with the hydroxy group of alcohol 592 to give the mixed orthoesters 593 and then ketene

acetals 594, which undergo rearrangement to form the olefinic esters 595 in good yields (ca 90%) (equation 247). This general approach was later used for a series of biomimetic syntheses of various polycycles. For instance, the total synthesis of all-trans-squalene 597 from succinic dialdehyde 596, which is about 98% stereoselective for each double bond, is exemplified by the synthetic sequence of equation 248333. Further examples of this fruitful approach to the synthesis of steroid systems (equations 249 and 250) were described in numerous papers334 – 343 and reviews344,345.

An analogous method based on treatment of the carbohydrate allyl alcohol 598 with an orthoester followed by Claisen rearrangement (called here the ‘oxa-Cope rearrangement’) was employed for the preparation of sphingosines (equation 251)346. An interesting example of a tandem reaction consisting of Claisen rearrangement and a subsequent shift of a carbon – carbon double bond was described as ‘a silicon mediated homo-Claisen rearrangement’ (equation 252)326.

Along with the Claisen rearrangement, other related reactions are applicable for the preparation of natural products. For instance a [2,3]-Wittig rearrangement is one step in the stereospecific synthesis of HMG-CoA reductase inhibitor pravastatin347 and in the total synthesis of the HMG-CoA synthase inhibitor 1233A348 according to the general scheme (equation 253).

In order to prepare cis-jasmone, a route to -ketoaldehydes was developed by using a thio-Claisen rearrangement (equation 254)349. The same rearrangement is the basis of a methodology for the diastereoselective synthesis of some branched homoallylic amine derivatives350. The rearrangement occurs at room temperature (equation 255). Schroth and coworkers have investigated351,352 the stereochemistry and reaction conditions for the chemical transformations of 3-exo-30 -exo-(1R, 10 R)-bis-thiocamphor 601 as a versatile source of functionally different 3,30-bibornane derivatives. Compound 601 was obtained from (1R)-thiocamphor 599 with sodium hydride in benzene and subsequent oxidation with iodine in benzene. The intermediate disulfide 600 undergoes a spontaneous ‘dithio-Cope’ rearrangement to form 601 (equation 256). A similar thermal 602 ! 603 rearrangement was assumed to be one step in the preparation of heterocyclic 1,2-dithiine precursors for the synthesis of ‘dithioxothioindigo’ (equation 257)353.