Macrocycle contraction

An interesting use of the Camps quinoline synthesis is in the ring contraction of macrocycles. Treatment of 9 member ring 24 with sodium hydroxide in water furnished quinolin-4-ol 25, while 26 furnishes exclusively quinolin-2-ol 27 under the same reaction conditions (no yield was given for either reaction). The reaction does not work with smaller macrocycles. The authors rationalize the difference in reactivity based upon ground state conformation differences, but do not elaborate. 

For the formation of undesired ring-contracted (by)products during macrocyclic diene-ene RCM with second-generation catalysts, see Sect. 1.3.3... 

The naphthalene-derivatized seleno-ether macrocycles dinaphtho-[16]aneSe4> naphtho-[8]aneSe2 and naphtho-[12]aneSe3 have also been prepared. Hydrolysis of dinaphtho-[16]aneSe4 with sulfuric acid leads to formation of the ring contracted species naphtho-[8]aneSe2.103,104... 

A novel ring-contraction reaction which proceeds via an epi-sulfonium ion intermediate has been reported for the simple and regiospeciflc synthesis of mono-and symmetrically di-functionalized tetrathiamacrocycles, starting with the mono- or dichloro-substituted macrocycles that have one or two more ring atoms. 

Kricheldorf HR, Lee S-R, Schittenhehn N (1998) Macrocyclic polymerization of (thio) lactones - stepwise ring expansion contraction. Macromol Chem Phys 199 273-282... 

Macrocyclic lactones. Ireland and Brown1 have adapted the Eschenmoser contraction of sulfides to a synthesis of five- and six-membered lactones. An example is formulated in equation (1). A hydroxy thioamide is esterified to give a chloro ester, which is then treated in sequence with Nal and phosphine I. The inelhod can also be used for preparation of macrocyclic lactones under high-dilution... 

Sapphyrins (39) are metallated with divalent metal aoetates in the presence of sodium acetate (Scheme 109).274 For the Zn11 and Co11 complexes, structure (44) was proposed with the dianionic macrocycle coordinating to the metal ion as a tetradentate ligand. These metallosapphyrins experience ring contraction to porphyrin complexes under mass spectral conditions. 

Initial cyclizadons were effected by the addition of an enamine to an imidate ester, both groups being suitably located by ligand coordination.263 An analogous process can be carried out on a thioimidate but a sulfide is formed and removal of sulfur with consequent ring contraction yields the corrin (100).264 These two complementary routes can be effected with different metal ions, nickel(II), palladium(II) and cobalt(III) in the first route, zinc(II) in the second. Removal of zinc ions easily provides the free corrin macrocycle. These two routes are summarized in Scheme 64. The sulfide contraction route was used in the Eschenmoser-Woodward total synthesis of vitamin Bn-265... 

With the name corrinoid we will refer to a class of compounds which have a molecular skeleton similar to that of the cobalt complex present in Vitamin B12 of which the main characteristics are the direct link between two pyrroles and the fully saturated p positions. These major features afford a macrocycle which is more contracted than a porphyrin and without an aromatic n system. 

This section will not be concerned with the detailed description of the synthetic methods leading to the appropriate precursors we will limit our attention to the crucial step of the synthesis of corrinoids, i.e. the formation of the tetrapyrrolic ring. The corrinoid macrocycle has been synthesized following two different procedures the first one involves the cyclization of a proper linear precursor, while the second involves ring contraction of a porphyrinoid structure. 

Another example of a ring contraction reaction leading to a corrinoid structure is the preparation of a,p,y-triazatetrabenzcorrole [83] the formula of which is reported in Fig. 30. This new macrocyclic ring has been isolated as its Ge4+ derivative formed by the ring-contractive reaction of germanium phthalo-cyanine carried out in the presence of NaBH4 or H2Se. 

In Section 2.2.1-2.2.3, a brief discussion on the chemistry, properties and structure of contracted, isomeric, and inverted porphyrins has been presented. Here, a brief sketch of the structural diversity in expanded porphyrins is given. Expanded porphyrins represent those synthetic macrocycles in which pyrrole or heterocyclic rings possess connectivity through mcso-carbon bridges. The conjugation of the porphyrin has been... 

In corroles and other systems described above, the macrocyclic ring is contracted by leaving out one of the four porphyrinic meso bridges. A more radical modification, namely an omission of one the cyclic subunits, results in even smaller macro-cyclic rings, collectively termed triphyrins. Boron subphthalocyanines (65, Fig. 25), known for 35 years [191], are an archetype of triphyrin structure. However, their... 

Planar hydroporphyrins have intrinsically larger cores (i.e., distances between opposite nitrogen atoms) than porphyrins. Deviation from planarity leads to a reduction in core size. As a result, the energy barrier for contraction of the macrocyclic core of hydroporphyrins upon complexation to small metals (e.g., Ni(II) or low- or intermediate-spin Fe(III)] is small. Experimental results have been discussed in detail (85JA4207). 

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