Scheme 1. Structure of azaspiracid (1) and retrosynthetic analysis leading to FGHI ring framework 3 via key intermediate 2.
previously known agents of diarrhetic shellfish poisoning. [1] This seasonally occurring toxin displays an unusually complex molecular assembly Β± it harbors a total of nine rings, eight of which are part of acetal or ketal structures Β± namely, an azaspiro ring system fused to a 2,9-dioxacyclo[3.3.1]nonane ring and a trioxadispiroketal fused to a tetrahydrofuran ring. Adding to the serious challenge posed by such a molecular framework, the absolute stereochemistry of the molecule and the relative stereochemistry between the ABCDE and the ammonia on the sample was performed at room temperature; the subsequent removal of ammonia was carried out at 120 8C for 1 h in flowing pure nitrogen.
Catalytic reactions: Two catalytic reactions were used to characterize the catalytic performance of the prepared materials, and analyses of the catalytic products were carried out using GC-8A and GC-17A (Shimazu Co.) instruments equipped with TCD and FID detectors. Catalytic cracking of 1,3,5-triisopropylbenzene was performed by the pulse method. Samples were calcined at 600 8C for 5 h to burn off any residual organic template. The catalytic testing was performed according to the following standard conditions: catalyst mass: 0.051 g; reaction temperatures in the range of 250 Β± 320 8C (no thermal cracking); the ratio of catalyst to 1,3,5-triisopropylbenzene or isopropylbenzene at 0.4 mL per 0.051 g. Nitrogen was used as carrier gas; flow rate 0.92 mL s Γ1 .
The catalytic alkylation of isobutane with butene was investigated at 2 MPa by using a stainless-steel apparatus equipped with a one-through stainlesssteel flow reactor. Typical reactions were carried out with 0.5 g of catalyst and an isobutane/butene ratio of 12:1, and a 1-butene/2-butene ratio of 8:1; the WHSV was 9 h Γ1 at a reaction temperature of 25 Β± 100 8C.