I. Introduction 93 II. Triangulanes: Nomenclature, Classification, and Stereoisomerism 94 III. Synthesis of Triangulanes 96 A. Synthetic Methods to Assemble Oligospirocyclopropanes 96 B. Synthesis of Unbranched Triangulanes (UTs) 97 C. Synthetic Approaches to Branched Triangulanes (BTs) 99 D. Cycloannelated Triangulanes (CATs) 102 E. Along the Route to Cyclotriangulanes (CTs) 103 F. Substituted Triangulanes 104 G. Heterotriangulanes 108 IV. Selected Physical Properties of Triangulanes 109 A. Spectral Data 109 B. Bonding Properties and Molecular Structures 110 C. Thermochemical Properties 115 V. Chemical Transformations of Triangulanes 117 A. With Retention of the Skeleton 117 B. With Ring Opening 121 VI. Triangulanes with Properties of Potential Practical Applicability 130 VII. Conclusions and Further Perspectives 133 VIII. Acknowledgments 136 IX. References 136 † Dedicated to Professor Wolfgang Lu ¨ttke on the occasion of his 80th birthday.

The construction of highly complex organic molecules by the assembly of simple molecular "building blocks" is not at all a new concept in organic synthesis, as, for example, numerous Diels-Alder and other cycloadditions or condensation reactions show. What is new, however, is an immensely increased number of preparative methods for linking the building blocks on the one hand and an enormous variety of types and complexities of employable oligofunctional small molecules on the other. In particular, the numerous metal-mediated processes that have become routine in the synthetic laboratories around the world during the last two decades allow the connection of carbon atoms and heteroatoms of practically any type of hybridization. As far as the building blocks are concerned, not only do they span the wide spectrum from very simple units, such as acetylene, to DNA, a "molecular brick" once regarded as being intractable for molecular construction, but it has also become possible to combine fragments from completely different subdisciplines of organic chemistrysacetylenes and sugars, for example, or fullerenes and porphyrinssgiving rise to completely novel hybrid structures.It still remains a dream that anything can be accomplished, that any mental image of a molecule can be transformed into material reality, as a small list of so far unknown hydrocarbonssthemselves only a grain of sand in a mountain of (conceivable) structural complexitysshows: for example,