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The antimony thin films were synthesized in ultra-high vacuum (base pressure: 4 î 10 À11 mbar) by evaporation of Sb 4 molecules from a resistance-heated effusive oven (temperature: 330 8C; deposition rate: 0.1 nm s À1 ; Sb: 99.9999 %, Johnson-Matthey). The layer thickness is given in monolayers of Sb atoms. The MoS 2 substrates were prepared by cleavage according to the procedure given in reference [13]. The AuSb 2 surface alloy was prepared starting from Au(111). [14] On this substrate the surface alloy AuSb 2 with the (100) orientation forms spontaneously, if less than one monolayer of antimony is deposited at room temperature. [15] The beetle-type STM [16] is located in an analysis chamber directly attached to the preparation chamber. [17] To increase image contrast, the STM data were electronically differentiated directly during acquisition. Therefore the STM images appear as if they are illuminated from the side.

Methyl 2-chloro-2-cyclopropylideneacetate (2) and its spiropentane analogue 3 cycloadd to dihydroisoquinoline N-oxide (9), pyrroline N-oxide (12), and C-phenyl-N-methylnitrone (16) to give 5-spirocyclopropaneisoxazolidines in good yields (58-93%). The thermal behavior of the 5-spirocyclopropaneisoxazolidines is rather differentiated, depending strongly on the constitution of the nitrone and the solvent. As nitrone 9 has the tendency to undergo cycloreversion reactions, the ketoamide rearrangement products 20 and 21 from its cycloadduct derive from the thermodynamically favored 4-spirocyclopropaneisoxazolidine regioisomers formed after the cycloreversion process. In DMSO as solvent different rearrangement processes take place, leading to benzoindolizinones in modest yields (15-21%). The cycloadducts from 12 and 16 undergo a cyclopropyl to cyclobutyl ring enlargement facilitated by the presence of a chlorine substituent on the carbon alpha to the spirocyclopropane ring. Whereas these compounds from nitrone 16 demonstrated an unusual stability, those from nitrone 12 undergo a cascade rearrangement to yield indolizinone derivatives 34, 35 cleanly (73-83% yield). This overall transformation offers a new method for the synthesis of the indolizine skeleton.

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A series of 3-spirocyclopropane-tetrahydropyrid-4-ones has been synthesized by the method consisting of nitrone cycloaddition to bicyclopropylidene and thermal rearrangement of the adducts. Regioisomeric 5-spirocyclopropanetetrahydropyrid-4-ones and 5-spirocyclopropanedihydropyrid-4ones were instead obtained by cycloaddition of nitrones and nitrile oxides, respectively, to methylenespiropentane, followed by thermal rearrangement. Methylenedispiro[2.0.2.1]heptane gave, in turn, 5,6-bis(spirocyclopropane)dihydropyrid-4-ones. The new compounds were prepared as simple aza analogues of the cytotoxic natural products illudins and ptaquiloside in order to study their activity in cleaving a DNA plasmid. The activities shown by several of the compounds are moderate, but from a comparative qualitative analysis of the results a useful structure-activity relationship for this new class of compounds could be derived.