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1 Spiro Compounds: A Brief History

       Marta Meazza

       School of Chemistry, University of Southampton, Southampton, UK

      Policyclic molecules containing at least two rings joined together by a single atom, mostly a carbon atom, previously named spiranes, are called spiro compounds or spirocycles, and the single central atom is referred to as the spiro atom [1]. We should mention that apart from carbon, other elements such as nitrogen, phosphorus, and arsenic may represent the spiro atom.

      The term was coined by the German chemist Nobel laureate Adolf von Baeyer who created the first spirane in 1900 [2].

      This peculiar structural feature is present in natural products and has long been the subject of methodological studies and synthetic efforts [3].

      Several synthetic procedures for spiro compounds have been developed and will be extensively discussed in the next chapters. However, the asymmetric synthesis of spirocycles that allow the creation of stereogenic quaternary centers represent a demanding task for organic chemists. Even the concepts of spiro aromaticity and spiro antiaromaticity can be applied when spiroconjugation is possible [4].

The search for the key term “spiro” in SciFinderⁿ database, at the end of October 2019, resulted in more than 40 700 references with an exponential growth starting from the middle of the last century and an increasing attention to this subject is expected in the future (Figure 1.1).

      These massive research efforts cover a wide range of fields from organic and medicinal chemistry to material sciences and engineering, to name a few.

      The enormous interest in spiro compounds rely on their distinctive properties often associated with the three‐dimensional stereochemical features, reflecting on their pharmacological properties that include, among others, bactericidal, fungicidal, anticancer, cytotoxic, antidepressant, antihypertensive, insecticidal, herbicidal, and plant growth regulatory effects [5]. These properties are due to the tetrahedral nature of the spiro carbon and consequent asymmetric features associated with it.

Figure 1.1 Growing interest in spiro compounds in chemical literature.

Figure 1.2 Dye sensitizer 9,9‐spirobifluorene.

      Source: Lupo et al. [7].

In addition, many other practical utilizations include optoelectronic devices, ophthalmic lenses, and solar cells [6]. Compounds like 9,9‐spirobifluorene 1 (Figure 1.2) have application in dye‐sensitized solar cells (DSCs) and represent the most efficient alternative to the current solar cell technologies [7].

Spirocyclic compounds find technological application as efficient charge‐transfer molecules due to their intramolecular donor–acceptor structural feature amplified by spiroconjugation. The desired optical properties can be achieved by careful design of the spiro donor–acceptor characteristic as illustrated in Figure 1.3 [8]. When structural characteristics make it possible, spiro compounds can equilibrate with their non‐spiro analogues exhibiting photochemical phenomena like photochemical memory.

We report here some examples of carbocyclic and heterocyclic naturally occurring compounds containing the spiro moiety (Figure 1.4). One of the simplest compounds is the pheromone of the olive fly Dacus oleae5. Phelligridin G 6 from the fungus Phellinus igniarius has been long used in Traditional Chinese Medicine for the treatment of gonorrhea [9]. The antimycotic drug griseofulvin 7, isolated from a penicillium mold in 1939, found application in the treatment of fungal skin infections since 1957. Hecogenin 8, the aglycone part of a steroid saponin found in the plant Agave sisalana, is responsible for many therapeutic effects and is also used as a starting material in the synthesis of corticosteroids [10]. Horsfiline 9 is an oxindole alkaloid having analgesic effect, isolated from the plant Horsfieldia superba [11].

Figure 1.3 Donor–acceptor spiro compounds and colors displayed by them.

      Source: Wössner et al. [8].

Figure 1.4 Examples of naturally occurring compounds containing the spiro moiety.

A classic example of the importance of the presence of a spiro functionality is the retention of the biological activity of perhydrohistrionicotoxin 10, the completely reduced analogue of the potent nicotinic receptor antagonist alkaloid (−)‐histrionicotoxin 11, isolated from “dart‐poison” frogs, that clearly suggests the fundamental role of the spiropiperidine moiety in determining a strong receptor binding. The massive synthetic efforts on this topic are collected in a book chapter [12] (Figure 1.5).

Figure 1.5 Spiro functionality in nicotinic receptor antagonists.

      Source: Hart [12].

      As stated before, spirocycles

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