Exploded pericyclynes

To relieve some of the angle strain at the saturated vertices of the smallest pericyclynes, we decided to explore the big brother compounds with 1,3-diyne units along each side in place of the shorter alkyne linkages. In the same sense that [A pericyclynes can be thought of as exploded cycloalkanes with acetylene spacers inserted between the corners [9], the new family with 1,3-diyne spacers can be thought of as exploded pericyclynes. 

Besides doubling the number of sp-hybridized carbon atoms over which to spread the strain, this structural modification opened the door to completely different synthesis strategies. Furthermore, and perhaps equally importantly, it provided a convenient UV chromophore to use as a reporter for the similarities and differences in electronic properties among members of the family. 

The most obvious approach to such compounds is the one we call the shotgun synthesis [18, 22], in which a symmetrical, difunctional monomer is subjected to coupling conditions in the hope that some fraction of the growing chains will cyclize in competition with further chain elongation (Fig. 9-12) [30]. In fact, this approach actually works, albeit to only a modest degree. 

The same cyclic pentamer was prepared much more efficiently by the cross-coupling of a preformed trimer, 45, with a preformed dimer, 47 [22, 31]. This was accomplished by separately converting the trimer to the corresponding bis-cuprate, 46, and the dimer to the bis-bromoalkyne, 48, and then mixing the two in pyridine under dilute conditions (Fig. 9-13). In this way, the five-sided macrocycle, 44, could be isolated in the greatly improved yield of 54%. 

Stepwise preparations of the acyclic dimer and trimer of 3,3-dimethylpenta-l,4-diyne are outlined in Fig. 9-14 [22, 31]. These two compounds occupied pivotal positions in many of our synthetic routes to macrocycles containing 1,3-diyne units. 

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Section 9.3 moves next to exploded pericyclynes, in which each side of the polygon consists of a longer 1,3-diyne unit. This is followed by hybrid systems wherein some sides are alkynes and others are diynes, and we conclude with a return to [AG pericyclynes, introducing heteroatoms at one or more of their vertices. 

Figure 9-12 Shotgun synthesis of exploded pericyclynes (a) CUCI2, CuCl, pyridine, 4.2% of 44 isolated lesser amounts of 42 and 43 identified only by GC/MS.

In an attempt to enhance the through-bond interactions of the out-of-plane p-orbitals, we embarked on a collaborative venture with Professor A. de Meijere to synthesize exploded pericyclynes that were spirocyclopropanated at every comer. Fig. 9-17 shows how the six-sided and the nine-sided members of this family were prepared [24]. Noteworthy here are (1) the direct formation of a bromoalkyne, 60, from a 2,2-dibromovinyl group (2) the desilylation of a TMS-protected acetylene in the presence of a bromoalkyne and (3) the absence of the three-sided compound in the cyclization (contrast with Fig. 9-15). 

An X-ray crystal structure of the 18-membered ring compound, 63 [24], shows that it adopts a giant chair conformation not unlike that of cyclohexane, albeit slightly less puckered, with essentially linear diynes and internal angles of 115.5° at the comers. Unlike the permethylated derivatives of exploded pericyclynes, crystals of the perspirocyclopropanated derivative, 63, explode when struck [33]. 

Figure 9-17 The first perspirocyclopropanated exploded pericyclynes (a) n-BuLi, Et20, then DMF then HjO (b) Zn, PhjP, CBf4, 70% for 58 - 59 (c) /-BuOK, THF, 95% (d) KF 2H2O, DMF, 74% for 60 - 61 84% 62 (e) 2 equiv n-BuLi, EtjO, then CuCl, then combine with 2 equiv 60 in pyridine, 54% (f) CuCl, Cu(OAc>2, pyridine, 39% 63, 8% 64.

With precursors in hand and reactions all worked out for the construction of both pericyclynes and exploded pericyclynes, the temptation to prepare hybrid macrocycles containing simple alkynes along some of the edges and 1,3-diynes along the others eventually became irresistible. Fig. 9-19 shows the route to one such crossbreed, 68, a 23-membered ring with nine triple bonds (cf. Fig. 9-13) [22, 31]. 

Figure 9-13 A two-component route to an exploded [5]pericyclyne (a) n-BuLi, THF, then CuCl (b) n-BuLi, THF, then TsBr, 69% (c) combine in pyridine, 54%.

Oxidative cyclization of acyclic trimer 45 under high dilution conditions gave us the first rational synthesis of the exploded [3]pericyclyne, 42, which was isolated in 39% yield (Fig. 9-15) [18]. Varying amounts (up to 8%) of the cyclic hexamer, 54, were also obtained... 

This macrocycle was prepared to determine whether any difference in electronic properties could be detected between homoconjugated macrocycles having 4A -i- 2 electrons in both the out-of-plane and the in-plane 7t-systems, as 68 has, and macrocycles having 4N electrons in both the out-of-plane and the in-plane n-systems, as the exploded" [5]pericyclyne 44 has. The answer, at least as far as UV absorption spectroscopy is concerned, is definitely negative all five bands in the UV spectrum of 68 are virtually superimposable on those in the UV spectrum 44 ( 1.5 nm) [22, 31]. Hiickel s rule does not operate in these systems. 

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