Ladderane

In chemistry, a ladderane is an organic molecule containing two or more fused cyclobutane rings. The name arises from the resemblance of a series of fused cyclobutane rings to a ladder. Numerous synthetic approaches have been developed for the synthesis of ladderane compounds of various lengths. The mechanisms often involve photocycloadditions, a useful reaction for creating strained 4-membered rings. Naturally occurring ladderanes have been identified as major components of the anammoxosome membrane of the anammox bacteria Planctomycetes. In chemistry, a ladderane is an organic molecule containing two or more fused cyclobutane rings. The name arises from the resemblance of a series of fused cyclobutane rings to a ladder. Numerous synthetic approaches have been developed for the synthesis of ladderane compounds of various lengths. The mechanisms often involve photocycloadditions, a useful reaction for creating strained 4-membered rings. Naturally occurring ladderanes have been identified as major components of the anammoxosome membrane of the anammox bacteria Planctomycetes. Synthetic approaches have yielded ladderanes of varying lengths. A classification system has been developed to describe ladderanes based on the number of consecutitive rings. The length of the ladderane is described by the number in brackets that precedes the word 'ladderane'. This is equal to the number of bonds shared by two cyclobutanes (n) plus 1. A ladderane of 3 or more units can connect in a circle, forming a band, which can also be considered to be two interconnected parallel cycloalkane rings. These are called prismanes. Ladderanes have two types of stereochemical relationships. One describes the relative arrangement of hydrogen atoms at the fusion between two cyclobutane rings. These hydrogen atoms can be in either the cis- or trans- configuration. Trans-ladderanes have not been synthesized due to the ring strain in these compounds. The second stereochemical relationship describes the orientation of three consecutive cyclobutane rings, and therefore is only relevant to ladderanes of n ≥ 2. The two outer rings can be on the same face (syn-) or on the opposite face (anti-), of the center ring. Various synthetic methods have been used for the laboratory synthesis of ladderane compounds. The three major approaches are (1) dimerization of polyene precursors, (2) the stepwise addition, one or two rings at a time, (3) and oligomerization. Several examples of ladderane synthesis are outlined below. The dimerization of two cyclobutadienes can generate both the syn and anti ladderane products depending on the reaction conditions. The first step in forming the syn product involves the generation of 1,3-cyclobutadiene by treatment of cis-3,4-dichlorocyclobutene with sodium amalgam. The reactant passes through a metalated intermediate before forming 1,3-cyclobutadiene, which can then dimerize to form the syn-diene. Hydrogenation of the double bonds will form the saturated syn--ladderane. To generate the anti product, cis-3,4-dichlorocyclobutene is treated with lithium amalgam. The lithium derivative undergoes a C-C coupling reaction to produce the open dimeric structure. This intermediate reacts to form the anti-diene, which can be hydrogenated to form the final anti--ladderane product. A different synthetic approach developed by Martin and coworkers has allowed for the synthesis of -ladderanes. The initial step involves the formation of a -ladderane from the addition of two equivalents of maleic anhydride with acetylene. The remaining two rings are formed from the Ramberg–Bäcklund ring contraction.