NOVELTY - Producing (M1) fuel constituent from bioisoprene composition comprising chemically transforming portion of isoprene in bioisoprene composition to non-isoprene compounds involves subjecting the bioisoprene composition to heat or catalytic conditions for isoprene dimerization to produce isoprene dimer and then catalytically hydrogenating isoprene dimer to form saturated 10C fuel constituent; or partially hydrogenating the bioisoprene composition to produce isoamylene, dimerizing isoamylene with a mono-olefin selected from e.g. isoamylene to form dimate and completely hydrogenating dimate.    USE - For producing fuel constituent from bioisoprene composition, where the fuel constituent used in fuel composition (claimed), transportation fuels, and in fuel blends e.g. as fuel oxygenates for emissions reduction and as fuel modifiers e.g. as cetane boosters for diesel. The fuel constituents or additives includes cyclic isoprene dimers and trimers, linear isoprene oligomers, aromatic and alicyclic isoprene derivatives, and oxygenated isoprene derivatives.    ADVANTAGE - The method allows high isoprene yield per cell, high carbon yield, high isoprene purity, high productivity, low energy usage, low production cost and investment and minimal side reactions; provides efficient, large scale and biosynthetic process for isoprene production that provides an isoprene source for synthetic isoprene-based products such as rubber, and also provides a desirable, low-cost alternative to using natural rubber.    DETAILED DESCRIPTION - Producing (M1) fuel constituent from bioisoprene composition comprising chemically transforming substantial portion of isoprene in the bioisoprene composition to non-isoprene compounds involves subjecting the bioisoprene composition to heat or catalytic conditions for isoprene dimerization to produce isoprene dimer and then catalytically hydrogenating the isoprene dimer to form saturated 10C fuel constituent; or partially hydrogenating the bioisoprene composition to produce isoamylene, dimerizing the isoamylene with a mono-olefin selected from isoamylene, propylene and isobutene to form dimate and completely hydrogenating the dimate to produce fuel constituent. INDEPENDENT CLAIMS are included for the following:    (1) a system (S1) for producing a fuel constituent from a bioisoprene composition, where a substantial portion of the isoprene in the bioisoprene composition is chemically converted to non-isoprene compounds, comprising a bioisoprene composition and (a) (i) at least one chemical capable of dimerizing isoprene in the bioisoprene composition or a source of heat capable of dimerizing isoprene in the bioisoprene composition; and (ii) a catalyst capable of hydrogenating the isoprene dimer to form a saturated 10C fuel constituent; or (b) (i) a chemical capable of partially hydrogenating isoprene in the bioisoprene composition to produce an isoamylene, (ii) a chemical capable of dimerizing the isoamylene with mono-olefins selected from isoamylene, propylene and isobutene to form a dimate and (iii) a chemical capable of completely hydrogenating the dimate to produce a fuel constituent; and    (2) a fuel composition (C1) comprising a fuel constituent produced by the method (M1).