You can borrow the 15% from friends or family or use a self directed IRA.  

You can start with a regular SDIRA with a custodian handling payments and then once you are comfortable that you are well versed in the rules, you can even do a checkbook IRA and have the freedom of investing and writing your own checks from your IRA in connection with ownership and operation of the property. 

That said, I'm contemplating stepping away from my current career, and funding a C-Corp Real Estate Operating company with my 401(k) along with an IRA belonging to my wife.  

Depending on the amount of commission income you receive, this may be the first step to take that causes the highest impact on your tax return.  2) How a SEP IRA plan may help you out.  

I’m just about to join a syndicated real estate investment.I will personally guarantee a large loan.My question is:Are my retirement accounts (IRAs, 401K, 403B, TSP) protected?

We are new to owner financing but own a property under a self directed IRA and are considering all options.  

A financial advisor can help you work out the specifics of your situation but investing in your company's 401k and perhaps a Roth IRA are a good place to start. 

My investment was in SD IRA and now in big trouble to pay loan because management company stealing all money from current rental income.

@Anthony HerbertPretty much any self-directed IRA provider is going to be "safe".  

This makes a lot of sense to me and I've been doing this for years through the traditional routes of 401(K) and Roth IRA.