3.3- dimethylpenta-l,4-diyne

The symmetrical acyclic tetrayne 21 used as a precursor to these new pericyclynes was easily assembled by two different methods (Fig. 9-8) [22]. the lengthier of the two started with the familiar building block 8, whereas the shorter route relied on the more difficultly-accessible 3,3-dimethylpenta-l,4-diyne, 26. 

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. 

From the dianion of 3,3-dimethylpenta-l,4-diyne, we obtained [4]pericyclynes and [6]pericy-clynes bearing heteroatoms at alternate corners, both with sulfur and with phosphorus (Fig. 9-25) [22] however, silicon gave the [6]pericyclyne, 89, and the [8]pericyclyne, 90, accompanied by none of the [4]pericyclyne (Fig. 9-25) [22]. We have no rational explanation for the change in behavior observed in going from sulfur and phosphorus to silicon. 

Under a nitrogen atmosphere, 2.11 g (19.7 mmol) of 3,3-dimethylpenta-l,4-diyne (26) was dissolved in 250 mL of dry THF. The solution was cooled to 0°C, and 16.4 mL (41 mmol) of 2.5 M -butyllithium in hexane was added dropwise. The mixture was stirred for 30 min, and then 4.06 g (41 mmol) of cuprous chloride was added. After stirring for 1 h at room temperature, the solvent was removed (not to dryness dry copper acetylides are explosive -CAUTION ) with reduced pressure and replaced with 650 mL of dry, oxygen-free pyridine. Next, 10.06 g (41.4 mmol) of bromoalkyne 50 in 35 mL of THF was added with a syringe pump over 6 h at room temperature. Stirring was continued overnight at room temperature. The dark green solution was poured slowly into 600 mL of ice-cold 25 % aqueous hydrochloric acid, and the aqueous mixture was extracted with 2 x 300 mL of pentane. The... 

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