Cycloadditions three-membered ring formation

Scheme 6.6 Cycloaddition/cycloreversion cascade followed by an unusual three-membered ring formation developed by Wegner et al.

The use of diazocarbonyls with acid catalysis under metal-free conditions has been reviewed (45 references), covering 1,3-dipolar cycloadditions, rearrangements, three-membered ring formation and Mannich-type and aldol reactions. ... 

Another interesting variation of the 1,3-dipolar cycloaddition involves generation of 1,3-dipoles from three-membered rings. As an example, aziridines 7 and 8 give adducts derived from apparent formation of 1,3-dipoles 9 and 10, respectively.148... 

As for the regioselectivity of the nitrone cycloaddition to MCP and its alkyl or aryl derivatives, a tendency of the three-membered ring to end up at the 4-position of the final isoxazolidine ring clearly emerges from the experimental findings. This result is particularly noteworthy if compared to regiospecific formation of the 5,5-disubstituted isoxazolidines in the reactions of nitrones, not... 

In contrast, with the calicene 230 TCNE is attached to five- and three-membered rings in a more complicated cycloaddition mode giving rise to 496s With a series of other calicenes no cycloaddition, but formation of stable charge-transfer complexes was observed2 93 ... 

Acetylene dicarboxylate and maleic anhydride failed to react with simple methylene cyclopropenes, but reacted readily with calicene derivatives, as shown by Prinz-bach293. Thus ADD combined with benzocalicene 497 to give the dimethyl tri-phenylene dicarboxylate 499, whose formation can be rationalized via (2 + 2) cycloaddition across the semicyclic double bond as well as (4 + 2) cycloaddition involving the three-membered ring (498/501). The asymmetric substitution of 499 excludes cycloaddition of ADD to the C /C2 triafulvene bond (500), which would demand a symmetrical substituent distribution in the final triphenylene derivative. 

These cycloadditions lead to the formation of three membered rings commonly seen in the carbene addition to double bonds. 

The introduction of carbenes and carbenoids into synthetic organic chemistry revolutionized the synthesis of cyclopropane derivatives21. In particular, cyclopropanation of methylenecycloalkanes became a very useful method for the preparation of SPC. Moreover, since cycloaddition of carbenes to olefins involves a very fast concerted process (i.e. it eliminates any intermediates during the formation of the three-membered ring)21, the method is equally efficient for the preparation of both unstrained and highly strained compounds. 

One of the most fascinating reaction modes of disilenes is the ready formation of disiliranes, three-membered ring systems containing two silicon atoms and one heteroatom in the ring, which are hardly accessible by other routes. They are mostly formed by [2 + 1] cycloadditions of atoms or molecular fragments to the Si=Si double bond. One of the few examples of other accesses to this ring system is the reaction of AMithio-2,4,6-trimethylanilide 53 with the disilane 52, which affords the azadisilacyclopropane 54 in modest yield (equation 8)75. 

Silver compounds are versatile catalysts for various cycloaddition reactions, including [2 + 1]-, [2 + 2]-, [3 + 2]-, and [4 + 2]-cycloadditions. An example for the silver-catalyzed formation of three-membered rings by [2+ l]-cycloaddi-tion is the silacyclopropanation reaction of mono- and disubstituted alkenes by silylene transfer from the cyclohexene silacyclopropane 432 that was reported recently by Woerpel et /.355,355a (Scheme 127). The reaction tolerates a number of functionalities in the substrate (OBn, OSiR3, BuTlC, etc.,) and is stereospecific with regard to the cisjtrans... 

Synthesis of Three-Membered Rings with One Germanium or Tin Atom 1.09.7.1 Ring Formation by [2+1] Cycloaddition... 

Treatment of P3C3Bu 3 with [(CO)sW <—PMe], generated / si/u by thermal decomposition of 56b (R = Ph) at 110°C, furnished a 2 1 mixture of quadricyclane 58b and the tricyclic compound 59, which were separated by fractional crystallization as colorless or yellow crystals in 39% and 25% yield, respectively. The presence of a three-membered ring in 59 agrees with an initial attack of the phosphinidene complex at a P=C bond of the triphosphinine to give transient 60. An intramolecular electrophilic aromatic substitution furnished product 59, whereas rearrangement of 60 to tetraphosphanorbornadiene 57a and its intramolecular [2+2] cycloaddition would rationalize the formation of 58b (Scheme 22) <2001CEJ3545>. 

An interesting domino reaction of 2-chloro-2-cyclopropylideneacetates was achieved with a variety of cycloalkadienolates. Thus, in a one-pot procedure tricyclic compounds such as 27 were obtained in high yields from 25 (X = Cl), whereas bicyclic products 26 result from 25 (X = H, Me). These are obviously formed via a [4-1-2] cycloaddition. For X = Cl in 25 a subsequent y-elimination of lithium chloride results in the formation of an additional three-membered ring. An alternative mechanism via two consecutive Michael reactions or a carbene intermediate can also be envisaged, but appears to be less likely. For examples see Table 5. 

In this case, a reaction pathway similar to that used to explain the formation of Diels-Alder products (see Section 2.2.2.1.) can be assumed to be operating. A butadiene molecule which is generated from MCP in the coordination sphere of the nickel, formally by proximal addition of the Ni —H bond to the three-membered ring and subsequent 6-hydrogen elimination, then reacts with a second MCP molecule in a [3 + 2] cycloaddition. 

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