Donor-acceptor-substituted cyclopropane

Scheme 36 Synthesis of donor-acceptor-substituted cyclopropanes 165 and cyclopentenes 166 from complexes 163 and acceptor-substituted alkenes 164 [115,116]...

ReiBig, H.-U. Donor-Acceptor-Substituted Cyclopropanes Versatile Building Blocks in Organic Synthesis, 144, 73-135 (1987). 

The thermal [1] or photochemical [5] isomerization of N-silylated allylamine in the presence of Fe(CO)5 provides the corresponding N-silylated enamines 7a and 7b. Z-enamine 7b does not react in any of the examined cycloadditions. The cyclopropanation of E-enamine 7a with methyl diazoacetate under copper(I) catalysis provides the donor-acceptor-substituted cyclopropane 9 [1], which can be converted in good yield into the interesting dipeptide 10 [6]. 

Of great interest are the "donor-acceptor-substituted cyclopropanes" 19, such as 2-silyloxycyclopropanecarboxylic esters (19a), first reported by Reissig [19b] and then also studied independently by Marino [20]. The general reactivity pattern of cyclopropanes 19 is outlined in Scheme 5.15 [19a]. 

Fig. 4.22. Formation of 1,3-dipoles by ring fission of donor-acceptor-substituted cyclopropanes.

The reaction of heteroatom-substituted alkenes with electrophilic carbene complexes can lead to the formation of highly reactive, donor-acceptor-substituted cyclopropanes. This type of cyclopropane usually undergoes ring fission and rearrangement reactions under milder conditions than do unsubstituted cyclopropanes (Figure 4.22). 

Donor-Acceptor-Substituted Cyclopropanes Versatile Building Blocks in Organic Synthesis... 

Donor-Acceptor-Substituted Cyclopropanes — Principles of Synthesis and Ring... 

Not surprisingly, cyclopropanes became synthetic tools only after methods of their preparation had been improved. This is mainly due to progress in carbene chemistry, and several modes to construct donor-acceptor-substituted cyclopropanes also benefit from these advantages. Scheme 1 is an attempt to summarize the most important possibilities of how to build up the systems under discussion and it shows that four of six paths use a carbene or its equivalent. 

From the three direct [2 + l]-cycIoaddition routes, path a employing electronrich olefins and acceptor-substituted carbenes is the most efficient one, since the alkenes can be synthesized from carbonyl compounds or other precursors and the carbenes are produced from easily available diazo alkanes. Therefore this very flexible mode to construct donor-acceptor substituted cyclopropanes is by far the most frequently used route. 

The very first synthesis 9) of a donor-acceptor-substituted cyclopropane had been performed along route (f). Michael addition of a suitable nucleophile is followed by an intramolecular substitution (elimination of Hal-) to create the three membered ring. Since all carbon atoms are already assembled in the starting material, and the reaction is not very general, it did not receive too much attention. 

As early as 1938 Rambaud reported the first synthesis of a donor-acceptor-substituted cyclopropane — obtained by the addition/elimination path (0 (Scheme 1) — and he also recognized that these cyclopropanes are prone to ring cleavage providing 1,4-dicarbonyl compounds. After saponification 19 opens to 20 which is oxidized by air to the isolated succinic acid 219). 

The initial reports 39) only contain a few examples with unsaturated esters used as the olefinic component in large excess (entries 1-3, Table 1). Recent investigations 40), however, underline that this complementary approach to donor-acceptor-substituted cyclopropanes is rather general. Since equimolar amounts of olefins and carbene complexes are sufficient to give good results (entries 4-8), this method might be of preparative value. 

Table 1. Synthesis of Donor-Acceptor-Substituted Cyclopropanes with Fischer-Carbene-Complexes...

Exploiting these major advantages, siloxysubstituted cyclopropanes have become extremely versatile building blocks for many preparative purposes in the last decade. Besides these important practical aspects, they also have provided a deeper insight into many fascinating mechanistic features of donor-acceptor-substituted cyclopropanes. 

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