Analogs three-ring

Fu and co-workers have shown that the l-chloro-boracyclohexa-2,4-diene 97 reacts with 4-phenylpyridine at room temperature to provide the />-terphenyl analog 26 in 76% yield as an early example of a borabenzene-pyridine-type complex (Equation 6). The X-ray structure of 26, discussed in Section 7.14.3, shows the three rings not to be coplanar <1997OM1501>. Exposure of the l-chloro-boracyclohexa-2,4-diene 98 to a stabilized carbene results in formation of the complex 29 in 83% yield as an air- and moisture-sensitive colorless powder (Equation 7), crystals of which were grown from toluene the X-ray structure of 29 has been discussed in Section 7.14.3. The use of bulky lithium bases to deprotonate the bis(amido) or bis(alkoxy)boranes 19 allows for the formation of the dianionic 2,2 -diboratabiphenyl derivatives 20 (X = NHPh, OBn), the solid-state structures of which have been discussed (Equation 8) <2006CJC81>. 

The three-ring analogs of strigol were prepared from 64 or 65 and the sodium salt of the appropriate bicyclic oxymethylenelactone (36,39). Of the compounds prepared, the isomeric GR-7 (68) and GR-28 (W), were highly effective in promoting germination of the seeds... 

Although the 4-RAS have not been tested as extensively as the three-ring and two-ring analogs of strigol, GR-24 is particularly attractive from the standpoint of activity and stability and appears to be a prime candidate for field evaluation and further study toward optimizing and simplifying the synthetic procedure. 

GERMINATION STIMULATION ACTIVITY OF A THREE-RING ANALOG OF STRIGOL, GR-7 (6) ON PARASITIC WEEDS... 

The palladium-catalyzed acylation of siloxycyclopropane furnishes a 1,4-di-carbonyl compound. A C-C bond of the three-membered ring is cleaved by an electrophilic attack of a palladium(II) species [103]. An analogous electrophilic ring opening of siloxycyclopropane was induced by various so-called ligand free transition metals such as Ag+ and Cu2+ [104]. 

The H NMR room temperature spectrum of 37b reflects an absence of symmetry, where all three rings of the triptycene are nonequivalent. At 160 °C, but not at lower temperatures, however, some peak broadening of triptycene (but not helicene) resonances is observed, indicating somewhat less retarded rotation. Extrapolation, based on analogy to our earlier work with the molecular brake, suggests a coalescence temperature of about 220 °C, which corresponds to a AG of approximately 25 kcal mol-1, a value that is corroborated by the studies described below and which is reassuringly close to the calculated barrier (AH, not AG ) of 22 kcal mol-1. 

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